Multi-focal laser scanning bar code symbol reading system employing a scan data signal processor having a variable pass-band filter structure with frequency characteristics controlled by detecting the focal distance of the laser scanning beam producing the analog scan data signal being processed by said data signal processor

ABSTRACT

The effects of paper/substrate noise are significantly reduced in multi-focal zone laser scanning systems by processing analog scan data signals with a scan data signal processor having a plurality of pass-band filters and amplifiers that are automatically selected for passing only the spectral components of an analog scan data signal produced when a bar code symbol is scanned at a particular focal zone in the laser scanning system. Two or more different pass-band filter structures can be provided for use in the scan data signal processor, wherein each pass-band filter structure is tuned to the spectral band associated with a particular focal zone in the laser scanning system. When a bar code symbol is scanned by a laser beam focused within the first focal zone or scanning range of the system, the pass-band filter structure associated with this focal zone or scanning range is automatically switched into operation. Only spectral components associated with the produced analog scan data signal and noise existing over this pass-band are allowed within the analog signal processor. By virtue of the present invention, first and second derivative signals can be generated and processed to produce a corresponding digital scan data signal for use in subsequent digitizing and decode processing operations, without compromising system performance due to the destructive effects of thermal and substrate noise outside the spectral pass-band of interest for the bar code symbol being scanned.

RELATED CASES

The present application is a continuation of copending application Ser.No. 09/442,718 filed Nov. 18, 1999, which is a continuation ofapplication Ser. No. 09/241,930 filed Feb. 2, 1999, now U.S. Pat. No.6,422,467, which is a Continuation-in-Part (CIP) of: copendingapplication Ser. No. 09/157,778 filed Sep. 21, 1998, which is aContinuation-in-Part of application Ser. Nos. 09/047,146 filed Mar. 24,1998, now U.S. Pat. No. 6,360,947, 08/949,915 filed Oct. 14, 1997, nowU.S. Pat. No. 6,158,659; 08/854,832 filed May 12, 1997, now U.S. Pat.No. 6,085,978; and copending application Ser. No. 08/886,806 filed Apr.22, 1997, now U.S. Pat. No. 5,984,185, which is a continuation ofapplication Ser. No. 08/726,522 filed Oct. 7, 1996, now U.S. Pat. No.6,073,846, which is a continuation of application Ser. No. 08/573,949filed Dec. 18, 1995, now abandoned. Each said patent application isassigned to and commonly owned by Metrologic Instruments, Inc. ofBlackwood, N.J., and is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an improved laser scanning systemwherein first and second derivative signals derived analog scan datasignals generated therewithin are processed in an improved manner sothat the effects of thermal noise and substrate/paper noise alike areminimized in diverse laser scanning environments including, multiplefocal zone scanning systems and large depth-of-field scanning systemsalike.

2. Brief Description of State of the Art

Code symbol scanners are widely used in diverse environments forpurposes of object identification, data-entry and the like.

During operation of such machines, a focused light beam is produced froma light source such as a visible laser diode (VLD), and repeatedlyscanned across the elements of the code symbol attached, printed orotherwise fixed to the object to be identified. In the case of bar codescanning applications, the elements of the code symbol consists of aseries of bar and space elements of varying width. For discriminationpurposes, the bars and spaces have different light reflectivity (e.g.the spaces are highly light-reflective while the bars are highlylight-absorptive). As the laser beam is scanned across the bar codeelements, the bar elements absorb a substantial portion of the laserbeam power, whereas the space elements reflect a substantial portionthereof. As a result of this scanning process, the intensity of thelaser beam is modulated in accordance with the information structureencoded within the scanned bar code symbol. As the laser beam is scannedacross the bar code symbol, a portion of the reflected light beam iscollected by optics within the scanner. The collected light signal issubsequently focused upon a photodetector within the scanner whichgenerates an analog electrical output signal which can be decomposedinto a number of signal components, namely: a digital scan data signalhaving first and second signal levels, corresponding to the bars andspaces within the scanned code symbol; ambient-light noise produced as aresult of ambient light collected by the light collection optics of thesystem; thermal noise produced as a result of thermal activity withinthe signal detecting and processing circuitry; and “paper” or substratenoise produced as a result of the microstructure of the substrate inrelation to the cross-sectional dimensions of the focused laser scanningbeam. The analog scan data signal has positive-going transitions andnegative-going transitions which signify transitions between bars andspaces in the scanned bar code symbol. However, a result of such noisecomponents, the transitions from the first signal level to the secondsignal level and vice versa are not perfectly sharp, or instantaneous,as in the underlying digital scan data signal. Consequently, it isdifficult to determine the exact instant that each binary signal leveltransition occurs in the detected analog scan data signal.

It is well known that the ability of a scanner to accurately scan a barcode symbol and accurately produce digital scan data signalsrepresentative of a scanned bar code symbol in noisy environmentsdepends on the depth of modulation of the laser scanning beam. The depthof modulation of the laser scanning beam, in turn, depends on severalimportant factors, namely: the ratio of the laser beam cross-sectionaldimensions at the scanning plane to the width of the minimal bar codeelement in the bar code symbol being scanned, and (ii) the signal tonoise ratio (SNR) in the scan data signal processor at the stage wherebinary level (1-bit) analog to digital (A/D) signal conversion occurs.

As a practical matter, it is not possible in most instances to produceanalog scan data signals with precisely-defined signal leveltransitions. Therefore, the analog scan data signal must be furtherprocessed to precisely determine the point at which the signal leveltransitions occur.

Hitherto, various circuits have been developed for carrying out suchscan data signal processing operations. Typically, signal processingcircuits capable of performing such operations include filters forremoving unwanted noise components, and signal thresholding devices forrejecting signal components which do not exceed a predetermined signallevel.

One very popular approach for converting analog scan data signals intodigital scan data signals is disclosed in U.S. Pat. No. 4,000,397,incorporated herein by reference in its entirety. In this US LettersPatent, a method and apparatus are disclosed for precisely detecting thetime of transitions between the binary levels of encoded analog scandata signals produced from various types of scanning devices. Accordingto this prior art method, the first signal processing step involvesdouble-differentiating the analog scan data input signal S_(analog) toproduce a second derivative signal S″_(analog). Then the zero-crossingsof the second derivative signal are detected, during selected gatingperiods, to signify the precise time at which each transition betweenbinary signal levels occurs. As taught in this US Patent, the selectedgating periods are determined using a first derivative signalS′_(analog) formed by differentiating the input scan data signalS_(analog). Whenever the first derivative signal S′_(analog) exceeds athreshold level using peak-detection, the gating period is present andthe second derivative signal S″_(analog) is detected for zero-crossings.At each time instant when a second-derivative zero-crossing is detected,a binary signal level is produced at the output of the signal processor.The binary output signal level is a logical “1” when the detected signallevel falls below the threshold at the gating interval, and a logical“0” when the detected signal level falls above the threshold at thegating interval. The output digital signal S_(digital) produced by thissignal processing technique corresponds to the digital scan data signalcomponent contributing to the underlying structure of the analog scandata input signal S_(analog).

While the above-described signal processing technique generates a simpleway of generating a digital scan data signal from a corresponding analogscan data signal, this method has a number of shortcomings anddrawbacks.

In particular, thermal as well as “paper” or substrate noise imparted tothe analog scan data input signal S_(analog) tends to generatezero-crossings in the second-derivative signal S″_(analog) in much thesame manner as does binary signal level transitions encoded in the inputanalog scan data signal S_(analog). Consequently, the gating signalmechanism disclosed in U.S. Pat. No. 4,000,397 allows “false”second-derivative zero-crossing signals to be passed onto thesecond-derivative zero-crossing detector thereof, thereby producingerroneous binary signal levels at the output stage of this prior artsignal processor. In turn, error-ridden digital data scan data signalsare transmitted to the digital scan data signal processor of the barcode scanner for conversion into digital words representative of thelength of the binary signal levels in the digital scan data signal. Thiscan result in significant errors during bar code symbol decodingoperations, causing objects to be incorrectly identified and/orerroneous data to be entered into a host system.

Also, when scanning bar code symbols within large scanning fieldsvolumes having multiple focal zones, as taught in co-applicant's PCTInternational Patent Publication No. WO 97/22945 published on Jun. 26,1997, Applicants have observed that the effects of paper/substrate noiseare greatly amplified when scanning bar code symbols in the near focalzone(s) of the system, thereby causing a significant decrease in overallsystem performance. In the far out focal zones of the scanning system,Applicants have observed that laser beam spot speed is greatest and theanalog scan data signals produced therefrom are time-compressed relativeto analog scan data signals produced from bar code symbols scanned infocal zones closer to the scanning system. Thus, in such prior art laserscanning systems, Applicants have provided, between the first and seconddifferentiator stages of the scan data signal processor thereof, alow-pass filter (LHF) having cutoff frequency which passes (to thesecond differentiator stage) the spectral components of analog scan datasignals produced when scanning bar code elements at the focal zonefurthest out from the scanning system. While this technique has allowedprior art scanning systems to scan bar codes in the far focal zones ofthe system, it has in no way addressed or provided a solution to theproblem of increased paper/substrate noise encountered when scanning barcode symbols in the near focal zones of such laser scanning systems.

Thus, there is a great need in the art for an improved analog scansignal processing device and method which enables precise detection ofsignal level transistions in analog scan data signals produced whenscanning bar code symbols using a multi-focal zone laser scanningsystem, while mitigating the effects of thermal and paper noiseencountered when scanning bar code symbols in both the near and farfocal zones thereof.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

Accordingly, it is a primary objective of the present invention toprovide an improved laser scanning system, wherein first and secondderivative signals from analog scan data signals produced therewithinare processed so that the effects of thermal and paper noise encounteredwithin the system are significantly mitigated.

Another object of the present invention is to provide an improved scandata signal processor having improved peroformance throughout the depthof field of its host scanning system by automatically tuning the scandata signal processor to an optimum setting for the focal zone beingscanned at each moment of scanning operations.

Another object of the present invention is to provide an improved scandata signal processor, wherein a variable first derivative signalpass-band filter is employed having pass-band filter characteristicsthat are dynamically controlled by the focal distance of the laserscanning beam producing the analog scan data signal being produced.

Another object of the present invention is to provide an improved scandata signal processor, wherein a different pass-band filter isdynamically switched into operation for pass-band filtering the firstderivative of analog scan data signals produced by laser scanned barcode symbols within each predefined focal zone in the laser scanningsystem, in order to filter out spectral components of paper noiseresiding outside the frequency spectrum of the analog scan data signalscanned within the predefined focal zone.

Another object of the present invention is to provide an improved scandata signal processor that can be used in any flying-spot type lightbeam scanning system wherein the speed of the light beam spot variessignificantly over the depth of the scanning range of the system.

Another object of the present invention is to provide such an improvedscan data signal processor, which can be used within holographic laserscanning systems, polygonal-type laser scanning system as well as anyother type laser scanning systems having multiple focal zones or a largedepth-of-field.

Another object of the present invention is to provide an improved scandata signal processor, wherein a time-domain non-linear substrate noisefilter is employed before the first derivative signal generation stageof the processor so as to produce, as output, a substantially fixedzero-reference signal level whenever a signal level indicative of thesubstrate is detected, and the signal level of the analog scan datasignal whenever a signal level indicative of a bar code element (e.g.dark bar) is detected.

Another object of the present invention is to provide a scan dataprocessor having a bar code element detector for automatically enablingthe second-derivative zero-crossing detector employed therein.

Another object of the present invention is to provide such a signalprocessing method, wherein detection of second derivative signalzero-crossings is automatically activated (i.e. enabled) upon detectionof bar element data encoded within the analog scan data signal, therebypreventing the detection of zero-crossings in the second derivativesignal caused by thermal and paper noise during bar code scanningoperations.

Another object of the present invention is to provide such a signalprocessing method, wherein after automatically enabling the detection ofzero-crossings in the second derivative signal, second-derivativezero-crossing detection is automatically disabled after a predeterminedtime period, and automatically re-enabled after redetection ofsubsequent bar code elements in the same bar code symbol or insubsequently scanned bar code symbols.

Another object of the present invention is to provide such a signalprocessing method, wherein detection of zero-crossings in the secondderivative of analog scan data input signals is enabled only whendigital scan data elements encoded therein are detected.

Another object of the present invention is to provide an imporved methodof processing analog scan data signals, wherein gating signals forsecond derivative zero-crossing detection are automatically generatedonly when the bar code element data is detected in the analog scan datainput signal, thereby subsstantially improving the overall performanceof the signal processor in the presence of thermal and paper noise.

Another object of the present invention is to provide a novel signalprocessor for carrying out such a signal processing method withindiverse types of bar code scanning devices and scanning environments(e.g. where data element stitching is employed).

Another object of the present invention is to provide a novel scan datasignal processor, wherein a real-time bar code element detector is usedto automatically activate and deactivate a second-derivativezero-crossing detector in response to the detection of the presence andabsence of bar code element data encoded within the analog scan datainput signal, respectively.

Another object of the present invention is to provide a novel laserscanning bar code symbol reader, wherewithin the scan data signalprocessor of the present invention is embodied.

Another object of the present invention is to provide a multi-focal zonelaser scanning system which employs a scan data signal processor thatallows reading of bar code symbols having bar code elementssubstantially narrower than the beam cross-section of the laser scanningbeam.

Another object of the present invention is to provide a multi-focal zonelaser scanning bar code symbol reading system, in which the ratio of theminimum laser beam cross-section dimension (MBD) to the minimum barelement width (MBW) in each focal zone of the system is greater than orequal to 2.0.

Another object of the present invention is to provide a multi-focal zonelaser scanning system, wherein a scan data signal processor is usedhaving a higher overall signal-to-noise ratio (SNR), thereby requiringless modulation of the laser scanning beam during scanning operations,and decreasing the effective laser beam diameter (i.e beam spot size) ateach focal zone in the system, and thus increasing the bar code scanningresolution of the system.

Another object of the present invention is to provide a multi-focal zonescanning system, wherein without increasing or otherwise changing thelaser beam power characteristics, the length of each focal zone in thesystem can be increased to allow either more overlap between adjacentfocal zones, or a larger overall depth of field in the system.

Another object of the present invention is to provide a laser scanningsystem that has an increased depth of field without increasing the powerlevel of the laser scanning beams, or adding additional focal zones tothe system.

Another object of the present invention is to provide a laser scanningsystem which employs a scan data signal processor having a plurality offirst derivative signal pass-band filter structures that areelectronically-switched into operation in response to control signalsderived from information about the focal distance of the laser scanningbeam at each instant in time.

Another object of the present invention is to provide a multi-focal zonelaser scanning bar code symbol reading system, wherein each scan dataproducing channel includes a scan data signal processor which employs avariable first derivative signal pass-band filter dynamically controlledby the focal distance of the laser scanning beam producing the analogscan data signal being processed.

Another object of the present invention is to provide a laser scanningbar code symbol reader employing a variable first derivative signalpass-band filter structure having frequency characteristics that arecontrolled in a real-time manner by measuring the time duration ofbinary signal levels in digital scan data signals produced in responseto laser scanning bar code symbol elements located within the scanningrange of the system.

Another object of the present invention is to provide a laser scanningbar code symbol reader employing a variable second derivative signalpass-band filter structure having frequency characteristics that arecontrolled in a real-time manner by measuring the time duration ofbinary signal levels in digital scan data signals produced in responseto laser scanning bar code symbol elements located within the scanningrange of the system.

Another object of the present invention is to provide such a laserscanning bar code symbol reading system, wherein time measurement of barcode symbol elements is carried out using an application specificintegrated circuit (ASIC) chip that compares real-time measurement ofbinary signal levels in digital scan data signals with predeterminedtime measures thereof, stored in an EPROM or like device, for bar codesymbols of different resolutions and scanning distances from the barcode symbol reading system.

Another object of the present invention is to provide such a bar codesymbol reading system, wherein a rotating polygon-type mechanism is usedto scan the laser scanning beam over the scanning field or volume of thesystem.

Another object of the present invention is to provide such a bar codesymbol reading system, wherein a variable laser beam focusing mechanismand rotating polygon-type mechanism are used to produce an X-bar or likelaser scanning pattern at varying depths of focus over the scanningfield of the system.

Another object of the present invention is to provide such a laserscanning bar code symbol reading system mounted over a high-speedconveyor-belt system in order to identify packages, parcels and the liketransported therealong in a highly reliable manner.

These and other objects of the present invention will become apparenthereinafter and in the Claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the Object of the PresentInvention, the following Detailed Description of the IllustrativeEmbodiment should be read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a schematic representation of a multi-focal zone holographiclaser scanning bar code symbol reading system mounted over aconveyor-belt along with bar coded packages of various dimensions aretransported in a high-speed manner;

FIG. 1A is a schematic representation of the multi-focal zoneholographic laser scanning bar code symbol reading system of FIG. 1,showing its omnidirectional laser scanning pattern substantiallycontained within a 3-D scanning volume having five different focalplanes (FP1 through FP5) and five overlapping focal zones (FZ1 throughFZ5);

FIG. 1B is a plan view of the holographic laser scanning system of FIG.1, showing its holographic scanning disc rotatably mounted on theoptical bench of the system, and surrounded by four laser scanningstations, each consisting of a laser beam production module disposedbeneath the scanning disc, a parabolic light collection mirror disposedbelow the scanning disc, a beam folding mirror disposed on the outerperimeter of and above the surface of the scanning disc, aphotodetection and signal preamplification module disposed above thescanning disc, and a signal processing board supporting both analog anddigital scan data signal processing circuits thereon, including the scandata signal processor of the present invention shown in FIGS. 4 through4H4;

FIG. 1C is a partially cutaway schematic view of one laser scanningstation disposed about the holographic scanning disc mounted within theholographic laser scanning system shown in FIG. 1;

FIG. 1D is an elevated side view of the home pulse mark sensing moduledeployed beneath the outer edge portion of the holographic scanning discin the system shown in FIGS. 1 through 1C;

FIG. 1E is a plan view of the home pulse mark sensing module shown inFIG. 1D;

FIG. 1F is a detailed schematic diagram of a home-pulse detection andVLD driver circuit for use in the holographic laser scanning system ofFIG. 1;

FIG. 1G is a schematic representation of the holographic laser scanningdisc employed in the bar code symbol reading system of FIG. 1, showingits twenty holographic optical elements (HOEs) supported between thesupport plates of the scanning disc;

FIGS. 1H and 1H1, taken together, set forth a table containing designand construction parameters for each of the twenty holographic opticalelements (i.e. volume transmission holograms) supported on theholographic scanning disc in the holographic laser scanning system ofFIG. 1, as well as the laser beam speed at the center of each scanlinewithin each focal zone generated by the system, and the beam speed atthe maximum and minimum depths of field within each such focal zonetherein:

FIG. 1I is a table identifying the depth of field and the focal zoneswhich are included the “Near” and “Far” Scan Ranges of the holographiclaser scanning system shown in FIG. 1;

FIG. 2A is a graphical representation of the power spectrum of a nexemplary analog scan data signal produced when laser scanning a barcode symbol within the first focal zone (FZ1) of the zone holographiclaser scanning system of FIG. 1, shown plotted along with the powerdensity spectrum of the substrate noise signal produced while laserscanning the bar code symbol on its substrate within the first focalzone of the holographic laser scanning system of FIG. 1;

FIG. 2B is a graphical representation of the power spectrum of anexemplary analog scan data signal produced when laser scanning a barcode symbol within the second focal zone (FZ2) of the holographic laserscanning system of FIG. 1, shown plotted along with the power densityspectrum of the paper/substrate noise signal produced while laserscanning the bar code symbol on its substrate within the second focalzone of the holographic laser scanning system of FIG. 1;

FIG. 2C is a graphical representation of the power spectrum of anexemplary analog scan data signal produced when laser scanning a barcode symbol within the third focal zone (FZ3) of the holographic laserscanning system of FIG. 1, shown plotted along with the power densityspectrum of the paper/substrate noise signal produced while laserscanning the bar code symbol on its substrate within the third focalzone of the holographic laser scanning system of FIG. 1;

FIG. 2D is a graphical representation of the power spectrum of anexemplary analog scan data signal produced when laser scanning a barcode symbol within the fourth focal zone (FZ4) of the holographic laserscanning system of FIG. 1, shown plotted along with the power densityspectrum of the paper/substrate noise signal produced while laserscanning the bar code symbol on its substrate within the fourth focalzone of the holographic laser scanning system of FIG. 1;

FIG. 2E is a graphical representation of the power spectrum of anexemplary analog scan data signal produced when laser scanning a barcode symbol within the fifth focal zone (FZ5) of the holographic laserscanning system of FIG. 1, shown plotted along with the power densityspectrum of the paper/substrate noise signal produced while laserscanning the bar code symbol on its substrate within the fifth focalzone of the holographic laser scanning system of FIG. 1;

FIG. 2F is an exemplary graphical representation showing a superpositionof the power spectrums of paper noise and the analog scan data signalsproduced when laser scanning bar code symbols within the first, second,third, fourth and fifth focal zones of the holographic laser scanningsystem of FIG. 1;

FIGS. 3A1 through 3A3, taken together, show the subcomponents of theholographic laser scanning system of FIG. 1 configured together on theanalog signal preamplification boards, the analog/digital (decode)signal processing boards and the housing thereof;

FIG. 3B is a schematic representation of the start-of-facet—pulse (SFP)generator employed on each decode processing board in the holographiclaser scanning system of FIG. 1;

FIG. 3C is a first table containing parameters and information that areused within the SFP generation module of the SFP generator shown in FIG.3B2;

FIG. 3D is a schematic representation of the operation of thestart-of-facet pulse (SFP) generator employed within each SFP generatorof the holographic laser scanning system of FIG. 1, wherein start offacet pulses are generated within the SFP generator relative to thehome-offset pulse (HOP) received from the HOP generator on themother/control board in the system;

FIGS. 4—1 and 4—2, taken together, show a schematic diagram of the scandata signal processor of the present invention shown comprising atime-domain substrate noise filtering circuit, a first derivative signalgeneration circuit with focal-zone controlled first derivative signalpass-band filters and amplifiers integrated therewith, a secondderivative signal generation circuit including focal-zone controlledsecond derivative signal pass-band filters and amplifiers integratedtherewith, a first derivative signal threshold-level generation circuitfor generating upper and lower first derivative signal thresholds (usedin detected second derivative zero-crossing gating), a binary-level typeanalog-to-digital (A/D) signal conversion circuit having a bufferamplifier, a second derivative zero-crossing detector, a pair of firstderivative signal comparators and a digital output signal levelgeneration circuit, and also a bar code element detection circuit foractivating and deactivating the second derivative signal zero-crossingdetection circuit employed within the A/D signal conversion circuit;

FIG. 4A is a detailed schematic diagram of the time-domain substratenoise filtering circuit employed in the scan data signal processor ofFIGS. 4—1 and 4—2, shown comprising an analog signal amplifier, azero-reference signal generator, a clipping diode, a high-impedenceoutput amplifier and a buffer amplifier;

FIG. 4B is a graphical diagram of an analog scan data signal provided asinput to the time-domain substrate noise filtering circuit shown in FIG.4A, a zero-reference signal generated within the circuit, and thetime-domain filtered analog scan data signal produced as output from thetime-domain substrate noise filtering circuit;

FIG. 4C1 is a detailed schematic representation of the first derivativesignal generation circuit employed in the scan data signal processor ofFIGS. 4—1 and 4—2, shown comprising a first scan-range controlled firstderivative signal pass-band filter and amplifier arranged along thefirst channel (A) of the circuit corresponding to the “Near Scan Range”of the system, and a second scan-range controlled first derivativesignal pass-band filter and amplifier arranged along the second channel(B) of the circuit corresponding to the “Far Scan Range” of the system,as identified in FIG. 11;

FIG. 4C2 shows a graphical representation of the magnitude of thefrequency response characteristics of the first derivative signalpass-band filters realized by the differentiator circuit A and thelow-pass filter A employed along Channel A, and the differentiatorcircuit Band the low-pass filter B employed along Channel B of the firstderivative signal generation circuit of FIG. 4C1;

FIG. 4C3 is a table identifying the bandwidth of the first derivativesignal pass-band filters employed along Channels A and B of the firstderivative signal generation circuit of FIG. 4C1, as well as the controlsignal levels that enable and disable such pass-band filters duringsystem laser scanning operations;

FIG. 4C4 is a table setting forth approximation formulas for computingthe upper and lower cutoff frequencies f_(LA), f_(UA) and f_(LB),f_(UB), characteristic of the first derivative signal pass-band filtersemployed along Channels A and B, respectively, in the first derivativesignal generation circuit shown in FIG. 4C1;

FIG. 4C5 shows a graphical representation of the magnitude of thefrequency response characteristics of the first derivative signalpass-band amplifiers realized by the operational amplifier A employedalong Channel A, and the operational amplifier B employed along ChannelB of the first derivative signal generation circuit shown in FIG. 4C1;

FIG. 4C6 is a table identifying the bandwidth of the first derivativesignal pass-band amplifiers employed along Channels A and B of the firstderivative signal generation circuit of FIG. 4C1, as well as the controlsignal levels that enable and disable such pass-band filters duringlaser scanning operations;

FIG. 4C7 is a table setting forth approximation formulas for computingthe upper and lower cutoff frequencies f_(LA), f_(UA) and f_(LB),f_(UB), characteristic of the first derivative signal pass-bandamplifiers employed along Channels A and B, respectively, in the firstderivative signal generation circuit shown in FIG. 4C1;

FIGS. 4D1A and 4D1B, taken together, set forth a detailed schematicdiagram of the second derivative signal generation circuit employed inthe scan data signal processor of FIGS. 4—1 and 4—2, shown comprising afirst scan-range controlled second derivative signal pass-band filterand amplifier arranged along the first channel (A) of the circuitcorresponding to the “Near Scan Range” of the system, and a secondscan-range controlled second derivative signal pass-band filter andamplifier arranged along the second channel (B) of the circuitcorresponding to the “Far Scan Range” of the system, as identified inFIG. 1I;

FIG. 4D2 shows a graphical representation of the magnitude of thefrequency response characteristics of the second derivative signalpass-band filters realized by the differentiator circuit A and thelow-pass filter A employed along Channel A, and the differentiatorcircuit Band the low-pass filter B employed along Channel B of thesecond derivative signal generation circuit shown in FIG. 4C1;

FIG. 4D3 is a table identifying the bandwidth of the second derivativesignal pass-band filters employed along Channels A and B of the secondderivative signal generation circuit of FIG. 4D1, as well as the controlsignal levels that enable and disable such pass-band filters duringlaser scanning system operations;

FIG. 4D4 is a table setting forth approximation formulas for computingthe upper and lower cutoff frequencies f_(LA), f_(UA) and f_(LB),f_(UB), characteristic of the second derivative signal pass-band filtersemployed along Channels A and B, respectively, in the second derivativesignal generation circuit shown in FIG. 4D1;

FIG. 4D5 shows a graphical representation of the magnitude of thefrequency response characteristics of the second derivative signalpass-band amplifiers realized by the operational amplifier A employedalong Channel A, and the operational amplifier B employed along ChannelB of the second derivative signal generation circuit shown in FIG. 4D1;

FIG. 4D6 is a table identifying the bandwidth of the second derivativesignal pass-band amplifiers employed along Channels A and B of thesecond derivative signal generation circuit of FIG. 4C1, as well as thecontrol signal levels that enable and disable such pass-band filtersduring laser scanning operations;

FIG. 4D7 is a table setting forth approximation formulas for computingthe upper and lower cutoff frequencies f_(LA), fUA and f_(LB), f_(UB),characteristic of the second derivative signal pass-band amplifiersemployed along Channels A and B, respectively, in the second derivativesignal generation circuit of FIG. 4D1;

FIG. 4E is a detailed schematic diagram of the pair of first derivativesignal threshold level circuits employed within the scan data signalprocessor of FIG. 4, for generating upper and lower threshold levelsused to determine when the first derivative signal has attained its peakpositive and negative values during the signal processing method of thepresent invention;

FIGS. 4F and 4F1, taken together, set forth a detailed schematic diagramof the binary (i.e. one-bit) A/D signal conversion circuit employed inthe scan data signal processor of FIGS. 4—1 and 4—2, shown comprising asecond derivative zero-crossing detector realized by a comparator and ahigh-input/low-output impedence amplifier (i.e. buffer), a pair of firstderivative signal comparators for comparing the first derivative signalwith the first derivative signal upper and lower threshold levels, anddigital output signal generating circuit realized by a set/reset latchcircuit consisting of four NAND gates arranged to produce as output, adigital output signal corresponding to the analog scan data signalprovided as input to the scan data signal processor of FIGS. 4—1 and4—2;

FIGS. 4G and 4G1, taken together, set forth a detailed schematic diagramof the bar code element detection circuit employed in the scan datasignal processor shown in FIGS. 4—1 and 4—2, comprising a zero-referencesignal generator, an analog signal level shifting circuit, a comparatorcircuit, and a second-derivative zero-crossing detector enable circuit;

FIG. 4H1 is a graphical representation of a pair of spaced apart“bar-type elements taken from an exemplary bar code symbol;

FIG. 4H2 is a graphical representation of an exemplary analog scan datasignal and a zero-reference signal produced by the zero-reference levelgeneration circuit within the bar code element detector shown in FIGS.4G and 4G1 when scanning the bar elements shown in FIG. 4H2;

FIG. 4H3 is a graphical representation of the output signal producedfrom the comparator circuit employed within the bar code elementdetection circuit shown in FIGS. 4G and 4G1;

FIG. 4H4 is a graphical representation of the digital output signalproduced from the second derivative zero-crossing detector enablecircuit shown in FIGS. 4G and 4G1;

FIG. 5A is a graphical representation of the laser scanning beamdiameter of a holographic laser scanning system employing a prior artscan data signal processor, shown plotted as a function of distance awayfrom its holographic scanning disc, graphically indicating the depthdimension of each focal zone defined by the distance between theoccurrence of 10 millimeter beam cross-sectional diameter measures takenabout a particular focal plane at which a laser scanning beam is focusedto its minimum beam cross-sectional diameter;

FIG. 5B is a graphical representation of the cross-sectioned diameter oflaser scanning beams produced by the holographic laser scanning systemshown in FIG. 1, and employing the scan data signal processor shown inFIGS. 4—1 and 4—2, shown plotted as a function of distance away from theholographic scanning disc shown in FIG. 1G, graphically indicating anincrease in the depth dimension of each focal zone in the holographiclaser scanning system;

FIG. 5C is a table setting forth the laser beam cross-sectionaldimensions at each of the five focal planes in the holographic laserscanning system of FIG. 1, as well as the minimum bar code element widththat can be detected by the scan data signal processor shown in FIGS.4—1 and 4—2 and thus readable by the system shown in FIG. 1;

FIG. 6 is a schematic representation of an alternative embodiment of amulti-focal zone laser scanning system in accordance with the presentinvention, shown comprising the scan data signal processor of FIGS. 4—1and 4—2, adapted to process analog scan data signals generated from arotating-polygon type laser scanning beam mechanism contained within itshousing, producing an X-bar or like scan pattern dynamically focusedalong a plurality of different focal zones in order to extend the depthof field and thus scanning range of the system;

FIG. 6A is a block schematic diagram of the laser scanning system shownin FIG. 6;

FIG. 6B is a schematic representation of a Beam Spot Speed Look-Up Tableused by the real-time bar code element width measurement processoremployed in the laser scanning system, of FIG.6;

FIG. 7 is a schematic representation of another alternative embodimentof a laser scanning system in accordance with the present invention,realized in the form of a hand-supportable laser scanning bar codesymbol reading device, shown comprising the scan data signal processorof FIGS. 4—1 and 4—2 adapted to process analog scan data signalsgenerated from a laser scanning beam mechanism contained within itshousing;

FIG. 7A is a block schematic diagram of the laser scanning system shownin FIG. 7;

FIG. 7B is a schematic representation of a Beam Spot Speed Look-Up Tableused by the real-time bar code element width measurement processoremployed in the laser scanning system, of FIG.7;

FIG. 8 is a schematic representation of yet another alternativeembodiment of a laser scanning system in accordance with the presentinvention, realized in the form of a fixed or portable projection-typelaser scanning bar code symbol reading device, shown comprising the scandata signal processor of FIGS. 4—1 and 4—2 adapted to process analogscan data signals generated from a laser scanning beam mechanismcontained within its housing;

FIG. 8A is a block schematic diagram of the laser scanning system shownin FIG. 8;

FIG. 8B is a schematic representation of a Beam Spot Speed Look-Up Tableused by the real-time bar code element width measurement processoremployed in the laser scanning system, of FIG. 9;

FIG. 9A1 shows a graphical representation of the magnitude of thefrequency response characteristics of the analog scan data signalpass-band preamplifier employed after each photodetector in the laserscanning system of FIG. 1;

FIG. 9A2 is a table identifying the bandwidth of the analog scan datasignal pass-band preamplifier employed after each photodetector in thelaser scanning system of FIG. 1, as well as the control signal levelsthat enable and disable the same during system laser scanningoperations; and

FIG. 9A3 is a table setting forth approximation formulas for computingthe upper and lower cutoff frequencies f_(LA), f_(UA) and f_(LB),f_(UB), characteristic of the analog scan data pass-band preamplifieremployed after pbotodetector in the laser scanning system of FIG. 1.

FIG. 10A1 shows a graphical representation of the magnitude of thefrequency response characteristics of the analog scan data signalpass-band preamplifier employed before the first derivative signalgeneration circuit of FIG. 4C1;

FIG. 10A2 is a table identifying the bandwidth of the analog scan datasignal pass-band preamplifier employed before the first derivativesignal generation circuit of FIG. 4C1, as well as the control signallevels that enable and disable the same during system laser scanningoperations; and

FIG. 10A3 is a table setting forth approximation formulas for computingthe upper and lower cutoff frequencies f_(LA), f_(UA) and f_(LB),f_(UB), characteristic of the analog scan data pass-band preamplifieremployed before the first derivative signal generation circuit shown inFIG. 4C1.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS OF THE PRESENTINVENTION

Referring now to the figure drawings, the Detailed Description of theIllustrative Embodiments of the present invention will now be describedin great detail with reference to the figure drawings, wherein likeelements shall be indicated by like reference numerals.

In general, the scan data signal processor of the present invention,illustrated in FIGS. 4—1 through 5C, can be used in virtually any laserscanning bar code symbol reading system having a scanning field orvolume in which beam spot speed of the laser scanning beam varies as afunction of distance away from the scanning mechanism.

For purposes of illustration, four exemplary laser scanning systems aredisclosed herein as examples of the types of systems in which the scandata signal processor of the present invention can be embodied.

In FIGS. 1 through 5C, the scan data signal processor of the presentinvention is shown embodied within a multi-focal zone holographic laserscanning system mounted over a conveyor belt system.

In FIGS. 6 and 6A, the scan data signal processor of the presentinvention is shown embodied within a multi-focal zone polygonal-typelaser scanning system also mounted over a conveyor belt system. Ingeneral, embodiments of this type will include fixed projection laserscanners, tunnel-type laser scanning systems, and the like.

In FIGS. 7 and 7A, the scan data signal processor of the presentinvention is shown embodied within a hand-supportable laser scanning barcode symbol reading device capable of producing a scanning field orvolume having multiple focal zones or a large scanning range over whichthe maximum laser beam spot speed is about twice or greater than theminimum laser beam spot speed therewithin. In general, embodiments ofthis type will include hand-supportable devices such as hand-held laserscanners, finger-mounted laser scanners, wand-type scanners as well asbody-wearable laser scanners. Examples of such bar code symbol scanningsystems are disclosed in U.S. application Ser. No. 09/204,176,incorporated herein by reference in its entirety.

In FIGS. 8 and 8A, the scan data signal processor of the presentinvention is shown embodied within a hand-supportable laser scanning barcode symbol reading device capable of producing a scanning field orvolume having multiple focal zones or a large scanning range over whichthe maximum laser beam spot speed is about twice or greater than theminimum laser beam spot speed therewithin. In general, embodiments ofthis type will include fixed-mounted and portable projection-type laserscanning bar code symbol reading systems and devices. Examples of suchbar code symbol scanning systems are disclosed in U.S. Pat. Nos.5,796,091 and 5,557,093, incorporated herein by reference in itsentirety.

In each such embodiment of the present invention, the primary functionof the laser scanning mechanism, however realized, is to produce a laserscanning field or volume in which bar code symbols can be scanned in areliable manner. In each illustrative embodiment of the presentinvention, the speed of the laser beam spot (or cross-section) along theextent of the scanned laser beam will vary over the depth of thescanning range of the system. The further the laser beam spot is awayfrom the laser scanning mechanism, the greater the laser beam spot speedwill be, based on well known principles of physics. A useful measure ofsuch beam spot speed variation is given by the ratio of (i) the maximumlaser beam spot speed within the scanning field of the system, to (ii)the minimum laser beam spot speed in the scanning system. Hereinafter,this spot speed variation measure shall be referred to as the “Max/MinBeam Spot Speed Ratio” of a laser scanning system.

The substrate, usually paper, on which a bar code is printed reflects asignal of varying power when scanned with a focused laser beam within agiven focal zone in the system. The laser light energy reflected (i.e.scattered) off the scanned code symbol is directed onto a photodetectorby way of light collection and focusing optics. The photodetectorconverts these optical signals into corresponding electrical signals.The signal components produced by scanning the bar code substrate areunwanted and therefore are described as noise. Since the substrate isusually paper, consisting of fibers having a random spatial structure,such unwanted noise signals are commonly referred to as paper orsubstrate noise. The signal output from the photodetector is referred toas an analog scan data signal S_(analog) comprising the desired bar codesignal component as well as the paper noise components.

As a bar code is scanned within a focal zone disposed further away fromthe scanner, the analog scan data signal produced at the photodetectoris increasingly compressed on the time-domain by virtue of the fact thatthe laser beam speed increases as a function of distance away from thelaser scanning mechanism. In accordance with Fourier Analysisprinciples, compression of the analog scan data signal (including itsnoise components) represented on the time-domain results in an increasein or shift of power to the higher spectral components of the analogscan data signal represented on the frequency-domain. Thus, thefrequency spectra of an analog scan data signal (including its noisecomponents) undergoes a positive frequency shift as the correspondingbar code symbol is scanned further away from the laser scanning system.This phenomenon is graphically illustrated in FIGS. 2A through 2F.Therefore, when scanning bar code symbols in a multi-focal zone laserscanning system, the bandwidth of the system must be sufficient tosupport the spectral components of analog scan data signals produced atthe photodetector by scanning bar code symbols at the different focalzones of the system.

In accordance with teachings of the present invention, the effects ofpaper noise are significantly reduced in multi-focal zone laser scanningsystems by processing the analog scan data signal with a scan datasignal processor having a plurality of pass-band filters (andamplifiers) that are automatically selected for passing only thespectral components of the analog scan data signal produced when a barcode symbol is scanned at a particular focal zone in the laser scanningsystem. In the case of a five focal zone scanning system, five differentpass-band filter structures can be provided within the scan data signalprocessor of the present invention. When a bar code symbol is scanned bya laser beam focused within the first focal zone, the pass-band filterstructure associated with the first focal zone would be automaticallyswitched into operation in the signal processor so that only thespectral components associated with the produced analog scan data signaland noise present over this pass-band, are present within the analogsignal processor while the first and second derivative signals aregenerated and processed to produce a corresponding digital scan datasignal, substantially free from the destructive effects of thermal andsubstrate noise outside the spectral pass-band of interest for the barcode symbol being scanned.

Alternatively, in order to reduced the number of components required bythe scan data signal processor, the five focal zones can be divided intotwo or more scan ranges (e.g. Near Scan Range and Far Scan Range) sothat two or more pass-band filter structures are used to pass thespectral components of analog scan data signals produced when scanningbar code symbols within these predefined scanning ranges, whilerejecting the destructive power associated with spectral noisecomponents outside of the selected pass-band filter structure.

By virtue of the present invention, it is now possible to scan bar codesymbols in the nearest focal zone in a multi-focal zone system, andproduce first and second derivative signals (of the analog scan datasignal) that are substantially free of spectral noise components outsidethe pass-band filter structure associated with the nearest focal zone,so that the first and second derivative signals can be used in anoptimal manner to reliably generate a corresponding digital scan datasignal during A/D signal conversion within the signal processor. Theseand other advantages of the present invention will become apparenthereinafter.

Multi-Focal Zone Holographic Laser Scanning System of the PresentInvention

In FIGS. 1 and 1A, the scan data signal processor of the presentinvention is shown embodied within a multi-focal zone holographic laserscanning bar code symbol reading system 1 that is mounted over aconveyor belt system 2 for the purpose of reading bar coded packages asdescribed, for example, in Applicants' copending U.S. application Ser.No. 09/157,778 filed Sep. 21, 1998, Ser. No. 09/047,146 filed Mar. 24,1998, Ser. No. 08/949,915 filed Oct. 14, 1997, now U.S. Letters Pat. No.6,158,659, 08,854,832 filed May 12, 1997, now U.S. Letters Pat. No.6,085,978, 08/886,806 filed Apr. 22, 1997, now U.S. Letters Pat. No.5,984,185 and 08/573,949 filed Dec. 18, 1995, now abandoned, each ofwhich is incorporated herein by reference in its entirety.

As shown in FIGS. 1 and 1A, the holographic laser scanning system Iemploys a holographic scanning disc 3 shown in FIGS. 1B, 1C, 1D, 1E and1G to produce an omnidirectional scanning pattern within a well-defined3-D scanning volume during each revolution of the disc. In theillustrative embodiment, the holographic scanning disc 3 supports twentyholographic optical elements (i.e. facets) each of which is realized asa volume transmission hologram in the preferred embodiment. Methods fordesigning the holographic disc and its accompanying scanning platformare disclosed in great detail in copending U.S. application Ser. No.08/854,832 filed May 12, 1997, now U.S. Letters Pat. No. 6,085,978,08/886,806 filed Apr. 22, 1997, now U.S. Letters Pat. No. 5,984,185, and08/573,949 filed Dec. 18, 1995, now abandoned, supra. Design andconstruction parameters for the holographic scanning disc 3 are setforth in Table 1H. An optical layout for the facets on the scanning discis shown in FIG. 1G.

In the illustrative embodiment, the holographic laser scanning system 1comprises four laser scanning stations 4A through 4D arranged about theholographic scanning disc 3. Each time the holographic scanning discrevolves past the four laser scanning stations, the system generates anomnidirectional laser scanning pattern 5 within a highly confined 3-Dscanning volume 6 having five different focal zones which overlapslightly at the end portions of each such focal region. In FIG. 5B, agraph is presented which indicates the spatial dimensions of each focalzone in the system, as well as the beam diameter therewithin and theamount of overlap provided between adjacent focal zones in theholographic laser scanning system of the illustrative embodiment. Thespeed of the laser beams within each focal zone (FZ1, FZ2, FZ3, FZ4 andFZ5) is listed in the table of FIGS. 1H and 1H1, along side theparameters of the holographic scanning facet producing the laserscanning beam in the focal region. Notably, at the center of eachscanline in each focal zone, the beam speed is substantially equal.Also, from this table, it can be seen that the speed of the laser beamat the maximum depth of field in each focal zone is greater than thespeed of the laser beam at the center of the scanlines therein, whereasthe speed of the laser beam at the minimum depth of field in each focalzone is less than the speed of the laser beam at the center of thescanlines therein.

In order to reduce the number of pass-band filter structures requiredwithin the scan data signal processor of the holographic laser scanningsystem shown in FIG. 1, the five overlapping focal regions FZ1 thoughFZ5 are grouped into two distinct scanning ranges, referred to as theNear Scan Range and the Far Scan Range. The depth of field of each ofthese scanning ranges and the focal zones falling therewithin arespecified in FIGS. 1I and 5B.

As shown in FIG. 1B, the holographic scanning disc 3 is rotatablymounted on the bench 7 of the system, and is surrounded by laserscanning stations 4A through 4D. It is understood, however, that inalternative embodiments of the present invention more or less scanningstations can be provided about the holographic scanning disc in order tomodify the laser beam scanning pattern within the 3-D scanning volume ofthe system.

As shown in FIG. 1C, each laser beam scanning station comprises a numberof subcomponents including, for example: a laser beam production module8 disposed beneath the holographic scanning disc 3 for producing a laserbeam incident the underside surface of the scanning disc while havingrequisite beam characteristics described in detail in copending U.S.application Ser. Nos. 08,854,832 filed May 12, 1997, now U.S. LettersPat. No. 6,085,978, 08/886,806 filed Apr. 22, 1997, now U.S. LettersPat. No. 5,984,185 and 08/573,949 filed Dec. 18, 1995, now abandoned,supra; a beam folding mirror 9 disposed on the outer perimeter of thescanning disc and above its surface, for folding the focused laser beamdiffracted by the scanning disc; a parabolic light collection mirror 10disposed below the scanning disc for collecting laser light raysscattered off a scanned bar code symbol 11 (within the focal zoneassociated with the particular holographic facet), and transmittedthrough the corresponding scanning facet and subsequently focused andretransmitted through the scanning disc onto a focal point above thescanning disc, at which the photodetector 12 is supported by a PC-basedphotodetection and preamplification board 13, as shown; and a signalprocessing board 14 (14A through 14D) supporting analog and digitalsignal processing circuits thereon, including the scan data signalprocessor of the present invention. As shown in FIG. 1B, a control (i.e.mother) board 15 is mounted within the housing 16 of the scanningsystem. On the control board 15, interfaces and circuits are providedfor receiving and processing the digital scan data signals produced bythe signal processing boards 14A through 14D from the laser scanningstations in the system.

As shown in FIGS. 1B, 1D and 1E, the holographic laser scanning system 1also includes a home-pulse mark sensing module 17 mounted beneath theedge of the scanning disc 3 on the optical bench 7 of the system. Asshown in FIGS. 1D and 1E, the module 17 comprises a number ofsubcomponents, namely: a miniature module housing 18 supporting aphotodetector 19 which is connected to home pulse detection and VLDdrive and photodetection circuitry 20 supported on the control (mother)board 15; a visible laser diode (VLD) 21 for producing and passing alaser beam 22 through the edge of the scanning disc, onto thephotodetector 19; and VLD drive circuitry 23 also on control board, fordriving the VLD during scanner operation. The circuit 20 can beconstructed in accordance with the schematic diagram set forth in FIG.1F. During scanner operation, the ho me-pulse detector senses the homeindicator gap 21 on the scanning disc 3 each time the disc passes theindicator during disc rotation. The output of the home-pulse detectioncircuit 20 on the control board 15 is provided as input to amicroprocessor-based home offset pulse (HOP) generator 22 provided onthe control board, as shown in FIG. 3A2. As will be described in greaterdetail hereinafter, the HOP generator 22 generates home offset pulses(HOPs) based on the following information: the known angular width ofeach facet; the disc motor speed; and the sequence of facets on theholographic scanning disc. A different HOP is sent to each decode board14A-14D at a different instant in time corresponding to when the homeindicator gap 21 is presented before the incident scanning beam of thecorresponding scanning station about the disc. On the correspondingdecode board, a microprocessor-based start of facet pulse generator 23uses the received HOP and information about the disc speed, facetsequence, and facet angular width to generate a start of facet pulse(SFP) for each facet as it moves by the corresponding scanning station.

In the illustrative embodiment, the SFP can be used to generate a signalindicating which scanning range (e.g. Near Scan Range or Far Scan Range)is being scanned by the particular facet in use at any given time, andprovide that signal to the scan data signal processor 25 on thecorresponding decode processing board 14, as shown in FIG. 3A1. If theNear Scan Range is being scanned, then a digital “low” level is producedas output from the microprocessor 26 on the corresponding decodeprocessing board 14, in accordance with the table set forth in FIG. 1I.If the Far Scan Range is being scanned, then a digital “high” level isproduced output from the microprocessor 26.

As shown in the system diagram of FIGS. 3A1 through 3A3, the holographiclaser scanning system of FIG. 1 comprises a number of subsystemcomponents, many of which are realized on the mother control board 15,and the preamplification boards 13A through 13D supported above thescanning disc, and the digital signal processing boards 14A through 14Dbeneath the scanning disc, shown in FIGS. 1B and 1C. For sake ofsimplicity, it will be best to describe these system components bydescribing the components realized on each of the above-describedboards, and thereafter describe the interfaces and interactiontherebetween.

As shown in FIG. 3A1, each analog scan data signal processing board 14Athrough 14D has the following components mounted thereon: an associatedphotodetector 12A (through 12D), realized as a silicon photocell, fordetection of analog scan data signals; and an analog signalpreamplification circuit 24A (through 24D for amplifying detected analogscan data signals to produce analog scan data signal D₁.

In the illustrative embodiment, each photodetector 12A through 12D isrealized as an opto-electronic device and each signal preamplificationcircuit 24A through 24D aboard the analog signal processing board isrealized as a discrete circuit. These circuits are suitably mounted ontoa small printed circuit (PC) board, along with electrical connectorswhich allow for interfacing with other boards within the scannerhousing. With all of its components mounted thereon, each PC board issuitably fastened to the photodetector support frame along itsrespective central reference frame, as shown in FIG. 1C.

In a conventional manner, a portion of the scattered light rays off ascanned bar code symbol are reflected along the same outgoing light raypath towards the holographic facet which produces the scanned laserbeam. These reflected light rays D₀ are collected by the scanning facetand ultimately focused onto the photodetector by the parabolic lightreflecting mirror 10A disposed beneath the holographic scanning disc.The function of each photodetector 12A is to detect variations in theamplitude (i.e. intensity) of optical scan data signal D₀, and producein response thereto an electrical analog scan data signal D₁ whichcorresponds to such intensity variations. When a photodetector withsuitable light sensitivity characteristics is used, the amplitudevariations of electrical analog scan data signal D₁ will linearlycorrespond to light reflection characteristics of the scanned surface(e.g. the scanned bar code symbol). The function of the signalpreamplification circuitry 24A is to pass-band filter and preamplify theelectrical analog scan data signal D₁, in order to improve the SNR ofthe output signal.

In the illustrative embodiment, each digital scan data signal processingboard 14A (through 14D) is constructed the same. On each of these signalprocessing boards, the following devices are realized: an analog scandata signal processing circuit 25A (through 25D) constructed inaccordance with the principles of the present invention; a programmabledigitizing circuit 24A (through 24D) realized as an ASIC chip;start-of-facet pulse (SFP) generator 23A (through 23D) realized as aprogrammable IC chip, for generating SFPs relative to home-offset pulses(HOP) generated by a start of facet pulse (SFP) generator circuit on themother board 15; an EPROM 28A (through 28D) for storing parameters andinformation represented, for example, in the tables of FIG. 3C; andprogrammed decode computer 26A (through 26D) is realized as a programmedmicroprocessor and associated program and data storage memory and systembuses, for carrying out symbol decoding operations in a real-timemanner. In the illustrative embodiment, the analog scan data signalprocessor, the programmed microprocessor, its associated memory andsystems buses are all mounted on a single printed circuit (PC) board,using suitable electrical connectors, in a manner well known in the art.

As will be described in greater detail hereinafter, the function of thescan data signal processor (25A through 25D) is to perform varioussignal processing operations on the analog scan data signal D₁ receivedfrom the photodetector 12A (through 12D) and ultimately convert theelectrical analog scan data signal D₁ into a corresponding digital scandata signal D₂ having first and second (i.e. binary) signal levels whichcorrespond to the bars and spaces of the bar code symbol being scanned.In practice, the digital scan data signal D₂ appears as a pulse-widthmodulated type signal as the first and second signal levels thereof varyin proportion to the width of bars and spaces in the scanned bar codesymbol.

The function of the programmable digitizing circuit 27A (through 27D) inthe illustrative embodiment is to convert the digital scan data signalD2, associated with each scanned bar code symbol, into a correspondingsequence of digital words (i.e. a sequence of digital count values) D₃representative of package identification data. Notably, in the digitalword sequence D₃, each digital word represents the time lengthassociated with each first or second signal level in the correspondingdigital scan data signal D₂. Preferably, the digital count values are ina suitable digital format for use in carrying out various symboldecoding operations. Reference is made to U.S. Pat. No. 5,343,027 toKnowles, incorporated herein by reference, as it provides technicaldetails regarding the design and construction of microelectronicdigitizing circuits suitable for use in the holographic laser scanningsystem of the present invention.

In bar code symbol scanning applications, the function of eachprogrammed decode computer (26A through 26D) in the illustrativeembodiment is to receive each digital word sequence D₃ produced from itsrespective digitizing circuit (27A through 27D), and subject it to oneor more bar code symbol decoding algorithms in order to determine whichbar code symbol is indicated (i.e. represented) by the digital wordsequence D₃, originally derived from corresponding scan data signal D₁detected by the photodetector associated with the decode computer. Inbar code symbol reading applications, in which scanned code symbols canbe any one of a number of symbologies, a bar code symbol decodingalgorithm with auto-discrimination capabilities can be used in a mannerknown in the art.

As shown in FIG. 3A2, the central processing (i.e. mother) board 15comprises a number of components mounted on a small PC board, namely: aprogrammed microprocessor 29 with a system bus and associated programand data storage memory, for controlling the system operation of theholographic laser scanner and performing other auxiliary functions;first, second, third, and fourth serial data channels 30A through 30D,for receiving serial data input from the programmable decode computers26A (through 26D) respectively; an input/output (I/O) interface circuit31 for interfacing with and transmitting symbol character data and otherinformation to data management computer system 32; home pulse detector20 and associated VLD driver circuits, realized as the electroniccircuit shown in FIG. 1F, for detecting the home pulse generated whenthe laser beam 22 (from VLD 21 in home pulse marking sensing module 17in FIG. 1D) is directed through home-pulse indicator gap (between FacetsNos. 5 and 12 shown in FIG. 1G) and sensed by photodetector 19; andhome-offset-pulse (HOP) generator 22 realized as an ASIC chip, forgenerating a set of four home-offset pulses (HOPs) in response to thedetection of each home pulse by circuit 20.

In the illustrative embodiment, each serial data channel 30A through 30Dis realized as an RS232 port, although it is understood that otherstructures may be used to realize the function performed thereby. Theprogrammed control computer 29 also produces motor control signals, andlaser control signals during system operation. These control signals arereceived as input by a power supply circuit 33 realized on the powersupply PC board. Other input signals to the power supply circuit 33include a 120 Volt, 60 Hz line voltage signal from a standard powerdistribution circuit. On the basis of the received input signals, thepower supply circuit produces as output, (1) laser source enable signalsto drive VLDs (14A through 14D), respectively, and (2) motor enablesignals in order to drive the scanning disc motor 34 coupled toholographic scanning disc 3.

As shown in FIG. 3B, each SFP generator (23A through 23D) comprises: aclock 35 for producing clock pulses (e.g. having a pulse duration ofabout 4 microseconds); a SFP generation module 36 for generating SFPsusing the table of FIG. 3C in accordance with the process depicted inFIG. 3D; and a control module 37 for controlling the SFP generator, andresetting the clock 39 upon each detection of a new HOP from the HOPgenerator on the mother control board 15 associated with the holographicscanning unit. As shown in FIG. 3A1, SFPs are provided to the analogscan data signal processor 25A by way of bus 28, or equivalent meansknown in the art. As mentioned hereinabove, the SFPs are used by theanalog scan data processor 25A (through 25D) to select particular filtercharacteristics which optimize SNR during A/D signal conversion.

Having described the structure and function of the major components ofthe holographic laser scanning system of FIG. 1, it is appropriate atthis juncture to now describe in greater detail the scan data signalprocessor of the present invention 25A (through 25D) with reference toFIGS. 4—1 through 5C.

Analog Scan Data Signal Processor of the Illustrative Embodiment of thePresent Invention

As shown in FIGS. 4—1 and 4—2, each scan data signal processor (25Athrough 25D) is similar in structure and function, and comprises anumber of subcomponents, namely: a time-domain substrate noise filteringcircuit 40, described in greater detail in FIGS. 4A and 4B; a firstderivative signal generation circuit 41 having scan-range controlledfirst derivative signal pass-band filters and amplifiers integratedtherewith, described in greater detail in FIGS. 4C1 through 4C7; asecond derivative signal generation circuit 42 having scan-rangecontrolled second derivative signal pass-band filters and amplifiersintegrated therewith, and described in greater detail in FIGS. 4D1through 4D7; a first derivative signal threshold-level generationcircuit 43 for generating upper and lower first derivative signal; abinary-type A/D signal conversion circuit 44; and a bar code elementdetection circuit 45. The structure and function of each of theseprocessor subcomponents will be described in greater detail hereinbelowwith reference to the first laser scanning station 4A, denoted by theletter A.

Non-Linear Time-Domain Substrate Noise Filtering Circuit

As shown in FIG. 4A, the non-linear time-domain substrate noise filter40 comprises, a number of subcomponents, namely: a linear voltagepreamplifer 46; a zero-reference signal generator 47, realized as peakdetection; a clipping diode 48; a high-output impedence amplifier (i.e.buffer) 49; and a buffer amplifier 50. As shown in FIG. 4A, the outputof the voltage amplifier 46 is provided to the input of both the zeroreference level generator 47 and the high-impedence amplifier 49. Theoutput of the zero reference level generator 47 is provided to oneterminal of the clipping diode 48, whereas the output of the high outputimpedence amplifier 50 is connected to the other terminal of theclipping diode, as shown. The input of the output buffer amplifier 50 isconnected to the output of the high-output impedence amplifier 49,whereas the output of the buffer amplifier 50 provides “quiet” outputversion of the analog scan data signal S₀(t).

The function of the voltage preamplifer 46 is to increase the voltageseparation between the two levels of the input analog scan data signalcorresponding to the bars and spaces of the bar code symbol. This isdone in order to provide finer clipping action given the fixed voltagedrop across the diode when it is forward biased. The zero referencegenerator and the high output impedence buffer each receive thisamplified analog scan data signal. When the zero reference voltage levelis at a voltage corresponding to a space element in a bar code symbol,it is greater than the output of the impedence-matching buffer plus theforward bias voltage drop of the clipping diode 48, and the clippingdiode 48 is forward biased causing the input to the buffer amplifier 50to be equal to the zero reference voltage minus the voltage drop acrossthe forward biased clipping diode 48. When the analog scan data signalis at a voltage level corresponding to a bar element, then the analogscan data voltage level is transferred through the high output impedenceamplifier 49 to the output terminal of the buffer amplifier 50. Theoperation of the time-domain based substrate noise filter 40 is shown bythe graphical plots set forth in FIG. 4B.

Notably, the nonlinear operation of the substrate noise filter 40described above tends to smooth or otherwise filter out substrate noisein the analog scan data signal during signal levels corresponding toscanning of substrates and bar code spaces realized on the substrate.However, the time-domain operation of the substrate noise filter 40 doesnot attempt to filter out the substrate noise during signal levelscorresponding to scanning of code symbol bar elements. Thus, thetime-domain substrate noise filter 40 performs, in essence, amultiplication operation on the analog scan data (i.e. the analog scandata signal is multiplied by zero during substrate-related signal levelsand unity during bar element related signal levels). The form of themultiplication function is a digital signal having transitions thatcorrespond approximately with the signal level transitions in the analogscan data signal. Consequently, in accordance with Fourier Analysis, thefrequency spectra of the analog scan data signal is mathematicallyconvoluted with the Fourier transform of this digital function, therebysmoothing out the spectral peaks associated with the substrate noise,and thus smoothing out the first time derivative of the analog scan datasignal. Thus, the time-domain substrate noise filter 40 operates toprepare the analog scan data signal in such a way that its first timederivative dS₁(t)/dt, once generated in the first derivative signalgeneration circuit 41, will have less random fluctuations therein due tothe absence of sharp peaks in the power spectra of the underlyingsubstrate noise signal, thereby contributing to an overall improvementin performance of the scan data signal processor of the presentinvention.

The First Derivative Signal Generation Circuit Having Scan-RangeControlled Pass Band Filters and Amplifiers Integrated Therewith

As shown in FIG. 4C1, the first derivative signal generation circuit 41of the illustrative embodiment comprises two channels, A and B,corresponding to the Near Scan Range and Far Scan Range of the laserscanning system. Along channel A of the first derivative signalgeneration circuit 41, a number of subcomponents are arranged in aserial manner, namely: a signal differentiator circuit 50A and a firstderivative signal low-pass filter (LPF) circuit 52A cooperating toprovide a first derivative signal pass-band filter structure 53A; and afirst derivative signal pass-band amplifier 54A tuned to operate like alow pass filter (LPF) in order to prevent the R₃C₂ element fromoperating as a second signal differentiator circuit within the firstderivative signal generation circuit 41; and an analog switch 55A,realized as a FET, for commuting the output of the pass-band amplifier53A to the output terminal 56 of the first derivative signal generationcircuit 41, in response to the generation of an enable signal ENABLE Aproduced by an analog switch controller circuit 57 which receives as itsinput signal, a variable pass-band (VPB) control signal 58 generated bythe microprocessor 26A shown in FIG. 3A1, described in detailhereinabove. Likewise, along channel B of the first derivative signalgeneration circuit 41, a number of subcomponents are arranged in aserial manner, namely: a signal differentiator circuit 51B and alow-pass filter (LPF) circuit 52B cooperating to provide a firstderivative signal pass-band filter structure 53B; and a first derivativesignal pass-band amplifier 54B tuned to operate like a low pass filter(LPF) in order to prevent the R₃C₂ element employed therein fromoperating as a second signal differentiator circuit within the firstderivative signal generation circuit 41; and an analog switch 55B,realized as a FET, for commuting the output of the pass-band amplifier54B to the output terminal of the first derivative signal generationcircuit 41, in response to the generation of an enable signal ENABLE Bproduced by the analog switch controller circuit 57 describedhereinabove.

In FIG. 4C2, the magnitude of the frequency response characteristics ofthe first derivative signal pass-band filter structure 53A aregraphically represented. The bandwidth of the first derivative signalpass-band filter structure 53A and the control signal levels forenabling and disabling the same during system operation are set forth inthe table of FIG. 4C3. Formulas for computing the upper and lower cutofffrequencies f_(LA), f_(UA) and f_(LB), f_(UB), characteristic of thefirst derivative signal pass-band filter structure 53A are set forth inthe table of FIG. 4C4.

In FIG. 4C5, the magnitude of the frequency response characteristics ofthe first derivative signal pass-band amplifier structure 54A aregraphically represented. The bandwidth of the first derivative signalpass-band amplifier 54A and the control signal levels for enabling anddisabling the same during system operation are set forth in the table ofFIG. 4C6. Formulas for computing the upper and lower cutoff frequenciesf_(LA), f_(UA) and f_(LB), f_(UB), characteristic of the firstderivative signal pass-band amplifier 54A are set forth in the table ofFIG. 4C7. In general, these cut-off frequencies can be determined usingthe techniques described above while recognizing that the pass-bandamplifiers 54A and 54B cannot function as differentiators.

Notably, in the illustrative embodiment of the present invention, theapproximation formulas set forth in the tables of FIGS. 4C3 and 4C6 forcomputation of the corner cut-off frequencies of the pass-band filterstructure 53A and amplifier 54A have been formulated through carefulrecognition of three interrelated facts of physics, namely: (1) that thespot speed of the laser scanning beam is increased at focal planes orover focal zones disposed at increased distances from the laser scanner;(2) that the analog scan data signal corresponding to the spatialstructure of a code symbol element is time-compressed as the code symbolelement is scanned at a distance further away from the laser beamscanning mechanism (e.g. holographic scanning disc); and (3) thattime-compression of a signal (such as the analog scan data signal) alongthe time domain results in expansion of the frequency spectra of thecorresponding time-domain signal on the frequency domain, based on theprinciple of Fourier Analysis that a multiplication operation on thetime-domain results in a convolution operation on the frequency-domain,and that a convolution operation on the time-domain results in amultiplication operation on the frequency-domain.

As a result of this increased laser spot speed phenomenon, the frequencyspectra of analog scan data signals produced during the scanning of barcode symbols is shifted in the direction of increased frequency for codesymbols in focal zones disposed greater distances away from the laserscanning system, than for code symbols in focal zones disposed closer tothe laser scanning system. Also, the power spectral distribution ofpaper noise produced from substrates is shifted in the direction ofincreased frequency for substrates scanned in focal zones disposedgreater distances away from the laser scanning system, than forsubstrates in focal zones disposed closer to the laser scanning system.

Thus, when scanning bar code symbols in the near focal zones of thesystem (i.e. FZ1 and FZ2), the spectral components of paper noiseresiding in the frequency bands associated with the farther out focalzones (i.e. FZ3, FZ4 and FZ5) are greatly amplified during the firststage of signal differentiation and would otherwise be added to thespectral components of the analog scan data signal if not filtered outin an appropriate manner. Thus, to prevent such spectral noisecomponents from degrading the first time derivative of the analog scandata signal produced by scanning a bar code symbol within the Near ScanRange, the microprocessor 26A in FIG. 3A1 generates a “low” VPBcontroller input level, as indicated in FIG. 4C3, so as to cause the VPBcontroller 57 to generate an ENABLE A signal therefrom. In turn, thisenables the first analog switch 55A to commute the output signal fromamplifier 54A to the output terminal of the analog switch circuit 56,and thus enabling channel A of the first derivative signal generationcircuit 41 which corresponds to the Near Scan Range of the laserscanning system. Then when a bar code symbol is scanned within the FarScan Range, the microprocessor 26A in FIG. 3A1 generates a “high” VPBcontroller input level, as indicated in FIG. 4C3, so as to cause the VPBcontroller 57 to generate an ENABLEB signal therefrom. In turn, thisenables the second analog switch 55B to commute the output signal fromamplifier 54B to the output terminal of the analog switch circuit 55,and thus enabling channel B of the first derivative signal generationcircuit 41 which corresponds to the Far Scan Range of the laser scanningsystem.

The Second Derivative Signal Generation Circuit with IntegratedFocal-Zone Controlled Pass Band Filters and Amplifiers

As shown in FIGS. 4D1A and 4D1B, the second derivative signal generationcircuit 42 of the illustrative embodiment, like the first derivativesignal generation circuit 41, comprises two channels, A and B,corresponding to the Near Scan Range and Far Scan Range of the laserscanning system. Along channel A of the second derivative signalgeneration circuit 42, a number of subcomponents are arranged in aserial manner, namely: a signal differentiator circuit 61A and alow-pass filter circuit 62A cooperating to provide a second pass-bandfilter structure 63A; and a pass-band amplifier 64A tuned to operatelike a low pass filter (LPF) in order to prevent the R₃C₂ element fromoperating as a third signal differentiator circuit within the secondderivative signal generation circuit 42; and an analog switch 65A,realized as a FET, for commuting the output of the pass-band filter 64Ato the output terminal of the second derivative signal generationcircuit 42, in response to the generation of an enable signal ENABLE Aproduced by an analog switch controller circuit 67 which receives as itsinput signal, a variable pass-band (VPB) control signal 68 generated bythe microprocessor 26A shown in FIG. 3A1, described in detailhereinabove. Likewise, along channel B of the second derivative signalgeneration circuit 42, a number of subcomponents are arranged in aserial manner, namely: a signal differentiator circuit 61B and alow-pass filter circuit 62B cooperating to provide a second pass-bandfilter structure 63B; and a pass-band amplifier 64B tuned to operatelike a low pass filter (LPF) in order to prevent the R₃C₂ elementemployed therein from operating as a third signal differentiator circuitwithin the second derivative signal generation circuit 42; and an analogswitch 65B, realized as a FET, for commuting the output of the pass-bandfilter 64B to the output terminal 60 of the second derivative signalgeneration circuit 42, in response to the generation of an enable signalENABLE B produced by the analog switch controller circuit describedhereinabove.

In FIG. 4D2, the magnitude of the frequency response characteristics ofthe second derivative signal pass-band filter structure 63A aregraphically represented. The bandwidth of the second derivative signalpass-band filter structure 63A and the control signal levels forenabling and disabling the same during system operation are set forth inthe table of FIG. 4D3. Formulas for computing the upper and lower cutofffrequencies f_(LA), f_(UA) and f_(LB), f_(UB), characteristic of thesecond derivative signal pass-band filter structure 63A are set forth inthe table of FIG. 4D4.

In FIG. 4D5, the magnitude of the frequency response characteristics ofthe second derivative signal pass-band amplifier structure 63A aregraphically represented. The bandwidth of the second derivative signalpass-band amplifier 64B and the control signal levels for enabling anddisabling the same during system operation are set forth in the table ofFIG. 4D6. Formulas for computing the upper and lower cutoff frequenciesf_(LA), f_(UA) and f_(LB), f_(UB), characteristic of the secondderivative signal pass-band amplifier 64B are set forth in the table ofFIG. 4D7. In general, these cut-off frequencies can be determined usingthe techniques described above while recognizing that thesecond-derivative signal pass-band amplifiers 64A and 64B cannotfunction as differentiators.

When scanning bar code symbols in the near focal zones of the system(i.e. FZ1 and FZ2), the spectral components of paper noise residing inthe frequency bands associated with the farther out focal zones (i.e.FZ3, FZ4 and FZ5) are greatly amplified during first stage ofdifferentiation and would otherwise be added to the spectral componentsof the analog scan data signal if not filtered out in an appropriatemanner. Thus, to prevent such spectral noise components from degradingthe second time derivative of the analog scan data signal produced whenscanning a bar code symbol within the Near Scan Range, themicroprocessor 26A in FIG. 3A1 generates a “low” VPB controller inputlevel, as indicated in FIG. 4D3, so as to cause the VPB controller 67 togenerate an ENABLE A signal therefrom. In turn, this enables the firstanalog switch 65A to commute the output signal from amplifier 64A to theoutput terminal of the analog switch circuit 65A, and thus enablingchannel A of the second derivative signal generation circuit 42 whichcorresponds to the Near Scan Range of the laser scanning system. Thenwhen a bar code symbol is scanned within the Far Scan Range, themicroprocessor 26A in FIG. 3A1 generates a “high” VPB controller inputlevel, as indicated in FIG. 4D3, so as to cause the VPB controller 67 togenerate an ENABLE B signal therefrom. In turn, this enables the secondanalog switch 65B to commute the output signal from amplifier 64B to theoutput terminal 66 of the analog switch circuit 65, and thus enablingchannel B of the second derivative signal generation circuit 42 whichcorresponds to the Far Scan Range of the laser scanning system.

Notably, the approximation formulas disclosed herein for determining theupper and lower cut-off frequencies of the first and second derivativesignal pass-band filters 53A,53B and 63A,63B are first-orderapproximation formulas based on a mathematical model of the laser beamscanning process and assumptions about the spatial structure of bar codesymbols, wherein (i) a bar code symbol of finite length (having minimumand maximum bar code element widths of W_(min) and W_(max),respectively) is scanned by (ii) a laser beam having a Gaussian beamwidth (measured at its 1/e^(e) intensity points) and a beam spotvelocity (v_(b)) relative to the bar code symbol; (iii) laser light raysreflected off the scanned bar code symbol are collected by a lightfocusing system having a predetermined focal distance; and (iv) thefocused laser light rays are detected with a photodetector located atthe predetermined focal distance from the light focusing system. Inaccordance with this process model, an arbitrary bar code symbol (havingminimum and maximum width elements) is scanned by a Gaussian laser beammoving at a beam spot speed (v_(b)) and can be represented by aconvolution integral function carried out on the spatial domain, whereinthe weighing (i.e. modulation) of the laser beam intensity and thefocusing of the reflected/scattered laser light rays onto thephotodetector is given by the modulation transfer function (MTF) of thelaser scanning system as described above which, for purposes of thepresent invention, shall be expressed in terms of the variable time (t),rather than the spatial dimension (x), using the intermediate variablebeam spot speed v_(b). Thus, the MTF as defined above is equivalent inform to the analog scan data signal produced from the photodetector ofthe system.

Having constructed a process model along the lines described above, anumber of different techniques can be then employed to determineacceptable values for the upper and lower cut-off frequencies for thepass-band filters used in the scan data signal processor of the presentinvention. Several different techniques which can be used will bedescribed hereinbelow.

In accordance with a first technique, based on numerical analysis,mathematical techniques such as the Discrete Fourier Transform (DFT) canbe used to compute a discrete frequency spectrum representation from adiscrete representation of the MFT. According to this proposed method ofanalysis, DFTs are computed for the first time derivative of a pluralityof MTFs which correspond to a plurality of laser-scanned bar codesymbols of varying code structure and resolution. Thereafter, thespectral power distributions of these first derivative functions can becarefully analyzed to arrive at upper and lower cut-off frequencies forfirst derivative signal pass-band filters 53A and 53B. When the cut-offfrequencies are optimally selected, the first derivative signalpass-band filters 53A and 53B will optimally reject the spectralcomponents associated with thermal and substrate noise within thesystem, while passing with minimal attenuation the balance of power inthe spectral components of the first derivative signal. The selection ofpass-band cut-off frequencies in this manner will allow the use of lowerfirst derivative signal threshold level values during the firstderivative signal peak detection stage of the scan data signal processorhereof, thereby increasing the dynamic range and SNR within this stageof the scan data signal processor, and thus improving the performance ofthe A/D signal conversion therewithin.

Thereafter, DFTs are computed for the second time derivative of the sameplurality of MTFs which correspond to the plurality of laser-scanned barcode symbols of varying code structure and resolution. Then, thespectral power distributions of these second derivative functions can becarefully analyzed to arrive at upper and lower cut-off frequencies forsecond derivative signal pass-band filters 63A and 63B. When the cut-offfrequencies are optimally selected, the second derivative signalpass-band filters 63A and 63B will optimally reject the spectralcomponents associated with thermal and substrate noise within thesystem, while passing with minimal attenuation the balance of power inthe spectral components of the second derivative signal. The selectionof pass-band cut-off frequencies in this manner will ensure thatsecond-derivative signal zero-crossing detection is carried out at amaximum SNR value, thus improving the performance of the A/D signalconversion within the scan data signal processor hereof.

In accordance with a second technique, based on analytical methods, amathematical expression can be derived for the MTF of a generalized barcode symbol having minimum and maximum bar code element widths. The MTFwould be expressed as multi-variable function dependent on the variousprocess variables including time (t) identified hereinabove. Havingobtained a mathematical expression for the MTF, the first timederivative of this MTF can be derived (i.e. computed) to provide a firstmulti-variable function which is representative of the first timederivative of the analog scan data signal detected at the photodetectorof the laser scanning system. Thereafter, the second time derivative ofthe MTF is derived to provide a second multi-variable function which isrepresentative of the second derivative of the analog scan data signaldetected at the photodetector of the laser scanning system.

The next step in the analytical method involves deriving (i.e.computing) the Fourier Transforms of the first and second timederivative functions of the MFT, expressed as multi-variable functionsdependent on the various process variables including frequency (f)identified hereinabove. The resulting Fourier Transforms provide arepresentation of the spectral components underlying the time-domainreconstitution of the first and second time derivatives of the MFT (i.e.photodetected analog scan data signal). However, it will be helpful toremember that the MFT has been derived by convolving a Gaussian laserbeam with a bar code symbol of finite length having minimum and maximumelement widths.

Assigning assumed values to the various process parameters in theprocess model (including the beam spot speed in the focal plane orscanning distance of interest, the minimum and maximum bar code elementwidths, etc.), one can obtain the Fourier Transforms for the first andsecond time derivatives for the MFT expressed solely as a function offrequency (f). Thereafter, the power spectral density (PSD) functionscan be derived for both the Fourier Transform of the first timederivative of the MFT, as well as the Fourier Transform of the secondtime derivative of the MFT. The PSD function for the first timederivative of the MFT, expressed as a function of frequency (f),represents the average power per unit bandwidth (in hertz) in the firsttime derivative of the analog scan data signal detected at thephotodetector. The PSD function for the second time derivative of theMFT, expressed as a function of frequency (f), represents the averagepower per unit bandwidth (in hertz) in the second time derivative of theanalog scan data signal detected at the photodetector. Then, using wellknown mathematical techniques, one can determine which band offrequencies (delimited by lower and upper cut-off frequencies f_(L) andf_(U), respectively) contain a predetermined percentage (e.g. 90%) ofthe total normalized power (i.e. 100%) contained in the first timederivative of the MFT to yield optimal lower and upper cut-offfrequencies for the first derivative signal pass-band filter assigned tothe assumed focal plane or scanning distance in the system. Thereafter,using the same mathematical techniques, one can determine which band offrequencies (delimited by lower and upper cut-off frequencies f_(L) andf_(U), respectively) contain a predetermined percentage (e.g. 90%) ofthe total normalized power (i.e. 100%) contained in the second timederivative of the MFT to yield optimal lower and upper cut-offfrequencies for the second derivative signal pass-band filter assignedto the assumed focal plane or scanning distance in the system. The abovemethod can be repeated for each focal plane or scanning distance in thesystem in order to provide the lower and upper cut-off frequencies ofthe pass-band filters to be employed in each channel of the scan datasignal processor, that has been preassigned to handle signals derivedfrom particular focal planes, zones, regions or scanning distances inthe laser scanning system under design.

In accordance with a third technique, one may determine suitable lowerand upper cut-off frequencies for each focal-zone/scan-range controlledpass-band filter structure of the present invention, using DiscreteFourier Transforms and related methods. According to this technique, themulti-variable MTF is evaluated by fixing all parameters, exceptfrequency (f), using assumed values for the process parameters. Notably,the beam spot speed parameter should be fixed to the value associatedwith the focal plane/zone or scanning range assigned to the pass-bandfilter under design. Then, the first and second time derivatives arecomputed for the discrete-time representation of the MTF so as toprovide a first discrete-time derivative of the MTF and asecond-discrete-time derivative of the MTF. Thereafter, using well knownsampling techniques, the Discrete Fourier Transform (DFT) can becomputed for the computed first and second discrete-time derivatives ofthe MTF. Then, using well known mathematical techniques, one candetermine which band of frequencies (delimited by lower and uppercut-off frequencies f_(L) and f_(U), respectively) contain apredetermined percentage (e.g. 90%) of the total normalized power (i.e.100%) contained in the first discrete-time derivative of the MFT toyield optimal lower and upper cut-off frequencies for the firstderivative signal pass-band filter assigned to the assumed focal planeor scan range in the system. Thereafter, using the same mathematicaltechniques, one can determine which band of frequencies (delimited bylower and upper cut-off frequencies f_(L) and f_(U), respectively)contain a predetermined percentage (e.g. 90%) of the total normalizedpower (i.e. 100%) contained in the second discrete-time derivative ofthe MFT to yield optimal lower and upper cut-off frequencies for thesecond derivative signal pass-band filter assigned to the assumed focalplane or scanning distance in the system. The above method can berepeated for each focal plane or scanning range in the system in orderto provide the lower and upper cut-off frequencies of the pass-bandfilters to be employed in each channel of the scan data signalprocessor, that has been preassigned to handle signals derived fromparticular focal planes, zones, regions or scanning distances in thelaser scanning system under design.

In accordance with a fourth technique, one may derive approximationformulas for computing suitable, although not necessarily optimal,values for the lower and upper cut-off frequencies for eachfocal-zone/scan-range controlled pass-band filter structure in the scandata signal processor of the present invention. This alternative method,while not mathematically rigorous in comparison with the other filterdesign methods detailed above, requires a deep understanding of theinteraction and functional-dependence (on both the time/space andfrequency domains) between the various parameters employed in theprocess model described hereinabove. When using this method, it ispossible to arrive at approximation formulas for the upper and lowercut-off frequencies for the first and second derivative signal pass-bandfilters 53A,53B and 63A,63B, respectively, expressed in terms of processparameters, for example: beam spot speed v_(b); the minimum bar codeelement widths W_(min); and maximum bar code element width W_(max).Notably, the approximation formulas set forth in the figure drawingshereof have provided marked improvements in dynamic range and SNR infirst derivative signal processing, as well as marked improvements inSNR in second derivative signal zero-crossing detection, therebyproviding a significant improvement in overall system performance, asreflected in the graphs of FIGS. 5A through 5C. When using thisapproach, the filter designer must understand a number of basic conceptswhich operate between the time domain and frequency domain such as, forexample: that a convolution operation between two functions on thetime-domain or spatial-domain, results in a multiplication operation onthe frequency-domain between the Fourier Transforms of those time orspatial domain functions; that a multiplication operation between twofunctions on the time-domain or spatial-domain, results in a convolutionoperation on the frequency-domain between the Fourier Transforms ofthose time or spatial domain functions; that increasing the beam spotspeed of the laser beam results in time-compression of the photodetectedanalog scan data signal; that decreasing the width of a bar code elementcauses a positive shift in frequency of the first and subsequentspectral components in its component frequency spectrum; that increasingthe width of a bar code element causes a negative shift in frequency ofthe first and subsequent spectral components in its component frequencyspectrum; that increasing the beam spot speed during laser scanning abar code symbol structure results in time compression of the resultinganalog scan data signal and thus in an expansion (or positive shift) inthe frequency spectrum thereof; that decreasing the beam spot speedduring laser scanning a bar code symbol structure results in timeexpansion of the resulting analog scan data signal and thus in acompression (or negative shift) in the frequency spectrum thereof; thata decrease in beam spot size during laser scanning operations results inan expansion of the frequency spectrum of the resulting analog scan datasignal; and that an increase in beam spot size during laser scanningoperations results in a compression of the frequency spectrum of theresulting analog scan data signal. Using these principles, the filterdesigner can synthesize approximation formulas for the pass-band filterstructures which can be refined through empirical measurement and timeand frequency domain analysis of the first and second time derivativesignals pass therethrough, and other forms of undue experimentation.

Regardless of which filter design method is employed, when the cut-offfrequencies are optimally selected, the first derivative signalpass-band filters 53A and 53B will optimally reject the spectralcomponents associated with thermal and substrate noise within thesystem, while passing with minimal attenuation, the balance of power inthe spectral components of the first derivative signal. The selection ofpass-band cut-off frequencies in accordance with the present inventionwill ensure that first-derivative peak detection is carried out withmaximum dynamic range and maximum SNR, thus improving the performance ofthe A/D signal conversion within the scan data signal processor hereof.

Similarly, when the cut-off frequencies are optimally selected, thesecond derivative signal pass-band filters 63A and 63B will optimallyreject the spectral components associated with thermal and substratenoise within the system, while passing with minimal attenuation thebalance of power in the spectral components of the second derivativesignal. The selection of pass-band cut-off frequencies in this mannerwill ensure that second-derivative signal zero-crossing detection iscarried out at maximum SNR, thus improving the performance of the A/Dsignal conversion within the scan data signal processor hereof.

The filter design methods described above can also be used to determinethe upper and lower cut-off frequencies for the pass-band amplifiers andpreamplifiers employed in the scan data signal processor of the presentinvention.

First Derivative Signal Threshold Level Generation Circuits

As shown in FIGS. 4F and 4F1, first derivative signal threshold levelcircuit 43 is realized as a pair of positive and negative peak detectioncircuits 70A and 70B, respectively. As shown, the first time derivativeof the analog scan data signal, denoted by S′(t)=d[S(t)]dt, is providedto both the positive and negative peak detection circuits 70A and 70B.The positive peak detection circuit 70A generates the upper thresholdlevel (UPPER_THRESHOLD) for use when detecting “positive peaks” in thefirst derivative signal using a first comparator in the A/D signalconversion circuit 44. The negative peak detection circuit 70B generatesthe lower threshold level (LOWER_THRESHOLD) for use when detecting“negative peaks” in the first derivative signal using a secondcomparator in the A/D signal conversion circuit 44.

Binary A/D Signal Conversion Circuit

As shown in FIGS. 4F and 4F1, the binary (i.e. one-bit) A/D signalconversion circuit 44 comprises a number of subcomponents, namely: asecond derivative zero-crossing detector 70; a second derivativezero-crossing gating circuit 71; and a digital output signal generationcircuit 72.

As shown, the second derivative zero-crossing detector 70 comprises: acomparator circuit 73, realized by an operational amplifier, and beingenabled and disabled by the digital output signal generated by the barcode element detector 45; and a high-input/low-output impedenceamplifier with unity gain (i.e. buffer) 74 connected to the comparatorcircuit 73 so as to prevent the bar code element detector 45 fromloading the reference voltage V_(ref) as detection of second derivativesignal zero crossings is enabled or disabled by the bar code elementdetector during scanner operation. The function of the comparatorcircuit 73 is to compare (i) the second derivative signalS″(t)=d²[S(t)]/dt² produced from the second derivative generationcircuit with (ii) a zero voltage reference (i.e. the AC ground level) inorder to detect the occurrence of each zero-crossing in the secondderivative signal.

Notably, in the absence of noise, the occurrence of each secondderivative zero-crossing indicates that the first derivative signal isundergoing a (positive or negative) peak which corresponds to the pointin the analog scan data signal where a signal level transition hasoccurred. However, in the real-world, noise is notorious for producingfalse zero-crossing detections within the second derivativezero-crossing detection circuit 70 described above. To reduce the numberof “falsely detected” zero-crossings produced by noise, the secondderivative zero-crossing gating circuit 71 is provided. As shown inFIGS. 4F and 4F1, this circuit is realized using a pair of analogcomparator circuits 75A and 75B. The function of the second derivativezero-crossing gating circuit 71 is to gate the digital output signalgenerating circuit 72, only detected second derivative zero-crossingswhich occur between alternating positive and negative peaks detected inthe first derivative signal by the analog comparators 75A and 75B.

As shown in FIGS. 4F and 4F1, the digital output signal generatingcircuit 72 is realized by four NAND gates 76A through 76D configured asa set/reset latch circuit. As shown, the output of the analogcomparators 75A and 75B are connected to the first input terminals ofNAND gates 76A and 76B. The second input terminal of NAND gate 76C andthe output terminal of NAND gate 76D are tied together to provide thedigital scan data output signal D₂, corresponding to the analog scandata signal D₁ provided as input to the scan data signal processor ofthe present invention. The basic principles described in detail aboveare used by the A/D signal conversion circuit 44 to determine when toproduce a high or low output signal level in the digital scan datasignal D₂ generated therefrom during scanner operation.

Bar Code Element Detection Circuit

As shown in FIGS. 4G and 4G1, the bar code element detector 45comprises: a zero-reference signal generation circuit 78; a levelshifting (i.e. biasing) circuit 79; an analog comparator circuit 80; anda zero-crossing enable signal generation circuit 81. In a bar code,logical zero would represent the spaces and the white areas precedingeach bar code element (e.g. bar). The electrical level corresponding tothe logical zero level is what the logical zero reference generator 78seeks to find in a continuous manner. Thus the function of this circuitis to determine the electrical analog equivalent of the paper/substratewhich forms a base line for reference with the output of the levelshifting circuit 79.

As shown, the analog scan data signal S₁(t) is provided to both thezero-reference level generation circuit 78 as well as to the levelshifter circuit 79. The level shifting circuit 79 (e.g. realized as asumming amplifier) merely adds a DC bias level to the original analogscan data signal for the purpose of offsetting it slightly so that theanalog signal is then offset with respect to the logical zero referenceand thus the zero reference is a slightly away from zero towards logicalone. The zero-reference voltage level provided as output from circuit 78is provided as one input to the digital comparator 80. Notably, thezero-reference level generation circuit 78 is substantially the same asthe one used in the substrate noise filter 40 described above. Thezero-reference signal produced by this circuit is provided to one inputof the comparator circuit 80. A graphical representation of these twosignals is shown in FIG. 4H2. In this example, the signal is produced inresponse to detection of the bar code elements shown in FIG. 4H1. Asshown in FIG. 4H3, the output of the level shifter circuit 79 isprovided as the second input to the comparator circuit 80. The output ofthe comparator circuit 80 is provided as input to the zero-crossingenable signal generation circuit 81 which is realized as a missing pulsedetector. The output of the comparator circuit 80 is a series of pulseswhich are somewhat representative of the bar code elements which appearon the analog signal. The comparator output pulse train is fed into theinput of the missing pulse detector 81 which is inactive on its outputwhen there are no pulses on the analog scan data signal. As shown inFIG. 4H4, as soon as a pulse appears on the analog scan data signalinput, the output of the missing pulse detector 81 goes active (i.e. itchanges states from, for example, low to high). In this example, thestate of the missing pulse detector 81 would stay high as long as thereare pulses present on the analog scan data signal input. There is acertain time delay T built into the missing pulse detector 81, set byits time constant described below, which determines the amount of timebetween the end of the pulse train and the time that the missing pulsedetector 81 outputs changes in state from active to inactive, or in thisexample, from high to low. In the illustrative embodiment shown in FIGS.4G and 4G1, the time constant of the missing pulse detector 81 is equalto T=(0.27)(R₈ ₈C₄ ₅). In practice, the time constant value of themissing pulse detector 81 is set to a value greater than the timeduration of the widest bar code element being scanned at the slowestbeam speed in the first focal zone (FZ1) of the laser scanning system.

During operation, the missing pulse detector 81 has a low output as longas the input is a logic 1. As soon as there is a transition from high tolow on the input of the missing pulse detector, the output immediatelyswitches states from low to high and holds that state as long as thereare transitions on the input signal within the time period describedabove. When the missing pulse detector detects that there have been notransitions for that specified time period, its output will changestates from high to low and it will wait for another transition on itsinput.

The output of the missing pulse detector 81 controls (i.e. enables ordisables) the second derivative zero-crossing detection circuit 73 viathe zero-crossing enable signal. In particular, the missing pulsedetector 81 enables the second derivative zero-crossing detector 73 bymoving the second derivative zero reference up to analog ground (e.g.2.0 volts), and disables its operation by moving the second derivativezero reference down to earth ground (e.g. 0 volts). Notably, the outputof the missing pulse detector 81 is a TTL output and an open drainoutput is needed at the zero-crossing detector 73 so an inverter is usedso that its output is fed into the input of a switching transistor (e.g.a field effect transistor) whose collector is then connected to thesecond derivative zero-crossing comparator 73 as a control signalZEROCROSSING_ENABLE.

Polygonal-Type Laser Scanning System Employing Dynamically-AdjustedLaser Beam Focus Mechanism and Scan Data Signal Processor of the PresentInvention

In FIGS. 6 and 6A, an alternative embodiment of the multi-focal zonelaser scanning system 85 is shown. As shown in FIG. 6A, laser scanningsystem 85 comprises a number of subcomponents, namely: a systemcontroller 86 (e.g. realized as a programmed microcontroller); a visiblelaser diode (or like device) 87 for generating a laser beam ofsufficient power; a variable focus lens subsystem 88, realizable usingmovable optical components translated relative to each other inreal-time manner in response to control signals produced by the systemcontroller 86, so as to vary the focal distance of the laser beam atdifferent focal planes (e.g. focal zones) within the system during laserscanning operations; a laser scanning mechanism 89, for scanning thevariably focused laser beam along a predefined scanning pattern (e.g.X-bar pattern) during scanner operation; a photodetector 90 fordetecting the intensity of laser light reflected off a scanned bar codesymbol and producing an electrical analog scan data signal correspondingto the structure of the scanned bar code symbol; a preamplificationcircuit 24 for preamplifying the analog scan data signal produced by thephotodetector 90; a scan data signal processor 25 as shown, for example,in FIGS. 4—1 and 4—2, for processing the preamplified analog scan datasignal D₁ and generating a digital scan data signal D₂ corresponding tothe analog scan data signal provided as input thereto; a digitizercircuit 27 for producing digital words D₃ representative of the timeduration of the first and second signal levels in the digital scan datasignal D₂; a decode processor 26 for processing the digital words D₃produced from the digitizer circuit 27 (e.g. using decode tables and barcode stitching techniques when using high-speed X-bar or like scanningpatterns), so as to decode the digital scan data signal and producesymbol character data string representative of the correspondinglaser-scanned bar code symbol; a real-time bar code element widthmeasurement processor (e.g. programmed microprocessor) 91 for real-timemeasurement of the first and second binary signal levels occurring inthe digital scan data signal D₂, as a bar code symbol is scanned atdifferent focal planes (or zones) within the system, and real-timecomparison with predetermined time duration measures stored in a BeamSpot Speed Look-Up Table 92 as shown in FIG. 6B (e.g. realized usingEPROM or like memory structures), so as to determine the correspondinglaser beam spot speed that would produce such measured time durationsfor a bar code symbol of a particular resolution, scanned at aparticular focal plane in the system; and a pass-band filter controlsignal generator (e.g. programmed microprocessor) 93 for producing VPBcontrol signals based on the determined beam spot speed of the laserscanning beam (at each particular instant in time), and providing suchVPB control signals to the preamplification circuit 24, and the firstand second derivative signal band-pass filters and amplifiers employedin scan data signal processor 25.

In general, the laser beam scanning mechanism 89 can be realized aseither a holographic scanning mechanism similar to that used in thesystem of FIG. 1, a polygon-type scanning mechanism of the general typedisclosed in U.S. Pat. No. 5,557,093, or other electro-mechanical orelectroacoustical scanning mechanism (e.g. based on Bragg or like cellstructures) capable of scanning one or more focused laser beams withinthe scanning field of the system.

In FIG. 6A, an exemplary Beam Spot Speed Table 92 is shown for n-numberof bar code symbol resolutions (i.e. specified by the predeterminedminimum element width (x) in the bar code symbol), wherein a premeasureddigital time-duration count for each minimum bar code element (x) isprovided in the table according to scanning distance (d_(i)) away fromthe scanning mechanism. As shown, for each scanning distance d_(I) thereis a predetermined laser beam spot speed associated with the laserscanning system. For a given laser scanning system, a Beam Spot SpeedTable is constructed by measuring the signal level durations of minimumbar code elements scanned at predetermined focal planes in the system,converting these time count measures to digital words (i.e. digitalnumbers), and recording these numbers in the column of a table, assignedto a particular bar code element width. During system operation, the barcode element width measurement processor 91 produces digital time counts(i.e. numbers) which are compared with the digital time counts stored inthe columns of the table of FIG. 6B. The output of this count comparisonprocess is a corresponding beam spot speed value which would producesuch a time count when a bar code symbol of a particular resolution isscanned at the corresponding scanning distance from. The VPB filtercontrol signal generator 93 uses the produced beam spot speed (or likemeasure) to determine the appropriate VPB control signal so that thepass-band frequency characteristics are set for each preamplifier,filter and amplifier in the scan data signal processor in accordancewith the principles of the present invention.

In an alternative embodiment, the variable focal lens subsystem 88 canbe replaced with a fixed lens focusing system, adapted to focus a laserbeam over a single predetermined focal region determined by the beamwaist characteristics and maximum beam spot size requirements in thesystem.

Hand-Supportable Laser Scanning System Embodying the Scan Data SignalProcessor of the Present Invention

In FIGS. 7 through 7B, an alternative embodiment of laser scanningsystem of the present invention 95 is shown having a hand-supportablehousing which, in general, may have any one of many possible formfactors. Examples of such form factors are disclosed in copendingapplication Ser. No. 09/071,512, now abandoned, incorporated herein byreference in its entirety.

As shown in FIG. 7A, laser scanning system 95 comprises a number ofsubcomponents, namely: a system controller 86 (e.g. realized as aprogrammed microcontroller); a visible laser diode (or like device) 87for generating a laser beam of sufficient power; a fixed or variablefocus lens subsystem 88′ realizable using movable optical componentstranslated relative to each other in real-time in response to controlsignals produced by the system controller 86, so as to vary the focaldistance of the laser beam at different focal planes (e.g. focal zones)within the system during laser scanning operations; a laser scanningmechanism 89, for scanning the variably focused laser beam along apredefined scanning pattern (e.g. X-bar pattern) during scanneroperation; a photodetector 90 for detecting the intensity of laser lightreflected off a scanned bar code symbol and producing an electricalanalog scan data signal corresponding to the structure of the scannedbar code symbol; a preamplification circuit 24 for preamplifying theanalog scan data signal produced by the photodetector 90; a scan datasignal processor 25 as shown, for example, in FIGS. 4—1 and 4—2, forprocessing the preamplified analog scan data signal D₁ and generating adigital scan data signal D₂ corresponding to the analog scan data signalprovided as input thereto; a digitizer circuit 27 for producing digitalwords D₃representative of the time duration of the first and secondsignal levels in the digital scan data signal D₂; a decode processor 26for processing the digital words D₃ produced from the digitizer circuit27 (e.g. using decode tables and bar code stitching techniques whenusing high-speed X-bar or like scanning patterns), so as to decode thedigital scan data signal and produce symbol character data stringrepresentative of the corresponding laser-scanned bar code symbol; areal-time bar code element width measurement processor (e.g. programmedmicroprocessor) 91 for real-time measurement of the first and secondbinary signal levels occurring in the digital scan data signal D₂, as abar code symbol is scanned at different focal planes (or zones) withinthe system, and real-time comparison with predetermined time durationmeasures stored in a Beam Spot Speed Look-Up Table 92′ as shown in FIG.7B (e.g. realized using EPROM or like memory structures), so as todetermine the corresponding laser beam spot speed that would producesuch measured time durations for a bar code symbol of a particularresolution, scanned at a particular focal plane in the system; and apass-band filter control signal generator (e.g. programmedmicroprocessor) 93 for producing VPB control signals based on thedetermined beam spot speed of the laser scanning beam (at eachparticular instant in time), and providing such VPB control signals tothe preamplification circuit 24, and the first and second derivativesignal band-pass filters and amplifiers employed in scan data signalprocessor 25.

In general, the laser beam scanning mechanism 89 can be realized aseither a holographic scanning mechanism similar to that used in thesystem of FIG. 1, a polygon-type scanning mechanism of the general typedisclosed in U.S. Pat. No. 5,557,093, or other electromechanical orelectro-acoustical scanning mechanism (e.g. based on Bragg or like cellstructures) capable of scanning one or more focused laser beams withinthe scanning field of the system.

In FIG. 7B, an exemplary Beam Spot Speed Table 92′ is shown for n-numberof bar code symbol resolutions (i.e. specified by the predeterminedminimum element width (x) in the bar code symbol), wherein a premeasureddigital time-duration count for each minimum bar code element (x) isprovided in the table according to scanning distance (d_(i)) away fromthe scanning mechanism. As shown, for each scanning distance d_(I),there is a predetermined laser beam spot speed v_(i) associated with thelaser scanning system. For a given laser scanning system, a Beam SpotSpeed Table is constructed by measuring the signal level durations ofminimum bar code elements scanned at predetermined focal planes in thesystem, converting these time count measures to digital words (i.e.digital numbers), and recording these numbers in the column of a table,assigned to a particular bar code element width. During systemoperation, the bar code element width measurement processor 91 producesdigital time counts (i.e. numbers) which are compared with the digitaltime counts stored in the columns of the table of FIG. 7B. The output ofthis count comparison process is a corresponding beam spot speed valuewhich would produce such a time count when a bar code symbol of aparticular resolution is scanned at the corresponding scanning distancefrom. The VPB filter control signal generator 93 uses the produced beamspot speed (or like measure) to determine the appropriate VPB controlsignal so that the pass-band frequency characteristics are set for eachpreamplifier, filter and amplifier in the scan data signal processor inaccordance with the principles of the present invention.

Fixed Projection-Type Laser Scanning System Embodying the Scan DataSignal Processor of the Present Invention

In FIGS. 8 through 8B, another alternative embodiment of laser scanningsystem 100 is shown in the form of fixed or portable projection-typelaser scanning system. As shown in FIG. 8A, laser scanning system 100comprises a number of subcomponents, namely: a system controller 86(e.g. realized as a programmed microcontroller); a visible laser diode(or like device) 87 for generating a laser beam of sufficient power, afixed focus lens subsystem 88″ for focusing the laser beam over apredetermined scanning range, specified by the beam waistcharacteristics of the laser beam and maximum beam spot size required bythe system; a laser scanning mechanism 89, for scanning the focusedlaser beam along a predefined scanning pattern (e.g. X-bar pattern)during scanner operation; a photodetector 90 for detecting the intensityof laser light reflected off a scanned bar code symbol and producing anelectrical analog scan data signal corresponding to the structure of thescanned bar code symbol; a preamplification circuit 24 for preamplifyingthe analog scan data signal produced by the photodetector 90; a scandata signal processor 25 as shown, for example, in FIGS. 4—1 and 4—2,for processing the preamplified analog scan data signal D₁ andgenerating a digital scan data signal D₂ corresponding to the analogscan data signal provided as input thereto; a digitizer circuit 27 forproducing digital words D₃representative of the time duration of thefirst and second signal levels in the digital scan data signal D₂; adecode processor 26 for processing the digital words D₃ produced fromthe digitizer circuit 27 (e.g. using decode tables and bar codestitching techniques when using high-speed X-bar or like scanningpatterns), so as to decode the digital scan data signal and producesymbol character data string representative of the correspondinglaser-scanned bar code symbol; a real-time bar code element widthmeasurement processor (e.g. programmed microprocessor) 91 for real-timemeasurement of the first and second binary signal levels occurring inthe digital scan data signal D₂, as a bar code symbol is scanned atdifferent focal planes (or zones) within the system, and real-timecomparison with predetermined time duration measures stored in a BeamSpot Speed Look-Up Table 92′ as shown in FIG. 8B (e.g. realized usingEPROM or like memory structures), so as to determine the correspondinglaser beam spot speed that would produce such measured time durationsfor a bar code symbol of a particular resolution, scanned at aparticular focal plane in the system; and a pass-band filter controlsignal generator (e.g. programmed microprocessor) 93 for producing VPBcontrol signals based on the determined beam spot speed of the laserscanning beam (at each particular instant in time), and providing suchVPB control signals to the preamplification circuit 24, and the firstand second derivative signal band-pass filters and amplifiers employedin scan data signal processor 25.

In general, the laser beam scanning mechanism 89 can be realized aseither a holographic scanning mechanism similar to that used in thesystem of FIG. 1, a polygon-type scanning mechanism of the general typedisclosed in U.S. Pat. No. 5,557,093, or other electromechanical orelectro-acoustical scanning mechanism (e.g. based on Bragg or like cellstructures) capable of scanning one or more focused laser beams withinthe scanning field of the system.

In FIG. 8B, an exemplary Beam Spot Speed Table 92′ is shown for n-numberof bar code symbol resolutions (i.e. specified by the predeterminedminimum element width (x) in the bar code symbol), wherein a premeasureddigital time-duration count for each minimum bar code element (x) isprovided in the table according to scanning distance (d_(i)) away fromthe scanning mechanism. As shown, for each scanning distance d_(I),there is a predetermined laser beam spot speed v_(i) associated with thelaser scanning system. For a given laser scanning system, a Beam SpotSpeed Table is constructed by measuring the signal level durations ofminimum bar code elements scanned at predetermined focal planes in thesystem, converting these time count measures to digital words (i.e.digital numbers), and recording these numbers in the column of a table,assigned to a particular bar code element width. During systemoperation, the bar code element width measurement processor 91 producesdigital time counts (i.e. numbers) which are compared with the digitaltime counts stored in the columns of the table of FIG. 8B. The output ofthis count comparison process is a corresponding beam spot speed valuewhich would produce such a time count when a bar code symbol of aparticular resolution is scanned at the corresponding scanning distancefrom the system. The VPB filter control signal generator 93 uses theproduced beam spot speed (or like measure) to determine the appropriateVPB control signal so that the pass-band frequency characteristics areset for each preamplifier, filter and amplifier in the scan data signalprocessor in accordance with the principles of the present invention.

Alternative Embodiments of the Present Invention

In order to further improve the performance of the laser scanning systemof the present invention, the preamplification circuit 24A (through 24D)on each analog signal processing board 13A (through 13D) in the system,as well as the front-end amplification circuit 46 within the substratenoise filter 40 shown in FIG. 4A, can be modified to include multiplesignal processing channels (e.g. channel A and channel B), in a mannersimilar to that done in the variable pass-band filter structure shown inFIG. 4C1. The benefit of this modification would be to rejectsubstantially all of the spectral components associated with substrateand paper noise that reside outside the frequency spectrum of the analogscan data signal scanned within a particular focal zone of the system,at each instant in time during scanner operation.

In such an alternative embodiment, the preamplification circuit 24A(through 24D), typically realized with discrete electronic components,would include preamplification circuitry along both of its channels Aand B. The preamplification circuitry along each of its channels A and Bwould include a high-pass filter (HPF) structure for cutting off the lowfrequency noise signal components, a low pass filter (LPF) for cuttingoff the high frequency noise signal components, and an analog switchcontrolled by a VPB control signal produced by an analog switchcontroller of the type shown in 4C1. Together, the HPF and LPF in themodified preamplification circuit would cooperate to provide apreamplification structure having a variable pass-band filter structureintegrated therewithin. The HPF and LPF along channel A would be tunedin accordance with the filter design criteria set forth in FIGS. 4C5through 4C7, to reject substrate and thermal noise without causing theHFPto carry out a time derivative function on the analog scan datasignal. Also, the VPB control signal generated by the microprocessor 26A(through 26D) on the corresponding decode signal processing board 14A(through 14D) would be provided to the analog switch controller in theresulting analog scan data signal preamplification/filtering circuits sothat the pass-band frequency response characteristics thereof aredynamically controlled by the focal zone (or scanning range) in whichbar code symbols are being scanned instant by instant, on a real-timebasis. Exemplary frequency response characteristics for these analogscan data signal pass-band amplifiers are illustrated in FIGS. 9A1 and9A2, whereas approximation formulas for the upper and lower cut-offfrequencies are set forth in FIG. 9A3.

In such an alternative embodiment, the front-end preamplificationcircuit 46 in the substrate noise filter 40 would include amplificationcircuitry along both of its channels A and B. The preamplificationcircuitry along each of its channels A and B would include a high-passfilter (HPF) structure for cutting off the low frequency noise signalcomponents, a low pass filter (LPF) for cutting off the high frequencynoise signal components, and an analog switch controlled by a VPBcontrol signal produced by an analog switch controller of the type shownin FIG. 4C1. Together, the HPF and LPF in the modified preamplificationcircuit would cooperate to an amplification structure having a variablepass-band filter structure integrated therewithin. The HPF and LPF alongchannel A would be tuned in accordance with the filter design criteriaset forth in FIGS. 4C5 through 4C7, to reject substrate and thermalnoise without causing the HPF to carry out a time derivative function onthe analog scan data signal. Also, the VPB control signal generated bythe microprocessor 26A (through 26D) on the corresponding decode signalprocessing board would be provided to the analog switch controller inthe resulting analog scan data signal amplification/filtering circuit sothat the pass-band frequency response characteristics thereof aredynamically controlled by the focal zone (or scanning range) in whichbar code symbols are being scanned instant by instant, on a real-timebasis. Exemplary frequency response characteristics for these analogscan data signal pass-band amplifiers are illustrated in FIGS. 10A1 and10A2, whereas approximation formulas for the upper and lower cut-offfrequencies are set forth in FIG. 10A3.

In addition, any and all other preamplification, amplification and/orfiltering circuits disposed between each photodetector 12A (through 12D)and the one-bit A/D signal conversion circuit 44 can be modified alongthe lines described above to incorporate a focal-zone controlledpass-band filtering structure in accordance with the principles of thepresent invention. Such modifications to the laser scanning system ofthe present invention will ensure that only spectral components withinthe spectral band of the analog scan data signal produced by thephotodetector 12A (through 12D) are utilized during the first and secondderivative signal generation and handling processes carried out withinthe scan data signal processor of the system. In turn, suchmodifications can be expected to improve the overall SNR along theanalog scan data signal processing channel and thus improve the bar codesymbol reading performance of the laser scanning system.

One may also desire to modify the bar code element detection circuit 45shown in FIGS. 4G and 4G1 and described hereinabove by removing itsmissing pulse detector 81, while retaining its output transistor (i.e.FET) 82, and connecting the gate terminal of the FET 82 directly to theoutput terminal of the comparator 80 in the bar code element detectioncircuit 45. The resulting bar code element detector would operate asfollows: when the signal level of the input analog scan data signal isbelow the zero-reference level, set by the zero-reference levelgeneration circuit 78, then the output of the output transistor 82 is alogical low signal level, thereby disabling the second derivativezero-crossing detection circuit 73; and when the signal level of theinput analog scan data signal is above the zero-reference level, set bythe zero-reference level generation circuit 78, then the output of theoutput transistor 82 is a logical high signal level, thereby enablingthe second derivative zero-crossing detection circuit 73.

An advantage provided by the above-described bar code element detectoris that, by virtue of its elimination of the missing pulse detector andits inherent time constant, the modified bar code element detector willonly generate a second derivative zero-crossing enable signal, and thusenable gating of detected zero-crossings to the digital output signallevel generation circuit 72, for a time period equal to the timeduration that the analog scan data signal level exceeds thezero-reference level. Thus, voltage spikes in the analog scan datasignal, due to substrate noise, poor quality bar code printing, or othermarks on the scan code symbol, will not enable the zero-crossingdetection circuit 73 beyond the time duration that the analog scan datasignal level exceeds the zero-reference level. However, for practicalreasons concerning ambient light signals, which cause a low frequencyamplitude modulation of the analog scan data signal, it may be desirableto use a bar code element detection circuit employing a missing pulsedetector.

While each of the illustrative embodiments described hereinabove hasmultiple focal zones with different depths of focus (DF), it iscontemplated that the scan data signal processor can also be used withlaser scanning systems having a single focal zone with a large scanningrange. In such alternative embodiments, a mechanism will be provided forautomatically determining the position or distance of the laser-scannedbar coded object from the scanning system, and generating variable passband (VPB) control signals for carrying out filter switching operationsin the scan data signal processor of the laser scanning system.

In the illustrative embodiments described hereinabove, the variablepass-band filter structure of the present invention has been has beenrealized as a pair of differently-tuned analog pass-band filter circuitsembedded within an analog signal switching system, thus providing adifferently tuned pass-band filter for analog scan data signals producedby laser scanning bar code symbols in either the Near Scan Range of thesystem, or in its Far Scan Range. It is understood, however, that therea number of alternative ways to practice the present invention. Severaldifferent modes for carrying out the present invention will be describedbelow.

For example, in laser scanning systems having a plurality of predefinedfocal zones or regions, an equal number of differently-tuned pass-bandfilter circuits can be provided within an analog signal switching systemso that, while the preamplified analog scan data signal to be processedis provided to each such pass-band filter circuit, the output of theoptimally-designed pass-band filter can be dynamically switched intooperation under microprocessor control during laser scanning operations,as taught hereinabove. In such alternative embodiments of the presentinvention, predefined scanning ranges, such as “Near Scan Range” and“Far Scan Range”, need not be used as the focal zone or region in whichanalog scan data signals are generated will directly determine whichoptimally-designed pass-band filter circuit should be automaticallyswitched into operation during laser scanning operations.

Alternatively, in laser scanning systems having a plurality ofpredefined focal zones or regions, a variable or tunable pass-bandfilter circuit can be provided, wherein certain resistance, capacitance,and/or inductance values (i.e. R, C, and L, respectively) can beelectronically-controlled by either electrically-active and/orelectrically-switchable components provided within the variablepass-band filter design. In such alternative embodiments of the presentinvention, the certain resistance, capacitance, and/or inductance valueswill be controlled by a microprocessor or microcontroller so that thepredetermined lower and upper cut-off frequencies are set for thepass-band filter circuit optimally designed for focal zone or regionfrom which each analog scan data signal is generated during laserscanning operations.

Alternatively, rather than using “analog-type” circuit technology forrealizing the subcomponents of the scan data signal processor (i.e. thevariable pass-band filter circuits, the analog signal switches, thefirst and second signal differentiation circuits, the first and secondderivative signal amplifiers, the first derivative threshold levelgenerators, the second-derivative zero-crossing detectors, the firstderivative signal comparators and the digital output level generationcircuit), it is understood that the scan data signal processing methodand apparatus of the present invention can be implemented usingfinite-impulse response (FIR) type digital filters carried out eitherwithin a programmed microcomputer or using one or more custom orcommercially available digital signal processing (DSP) chips known inthe digital signal processing art. When carrying out a digital-filterimplementation of the scan data signal processor of the presentinvention, the preamplified analog scan data signal D₁ is firstpass-band filtered and then time sampled at least two times the highestfrequency component expected in the analog scan data signal, inaccordance with the well known Nyquist criteria. Thereafter, eachtime-sampled scan data signal value is quantized to a discrete signallevel using a suitable length number representation (e.g. 64 bits) toproduce a discrete scan data signal. A suitable quantization level canbe selected in view of expected noise levels in the signal.

Thereafter, the discrete scan data signal is processed by a digital(FIR) filter implementation of the time-domain substrate-noise filter ofthe present invention. The discrete scan data signal output therefrom ispass-band filtered using an FIR-type digital filter whose filterparameters (e.g. time delays and multiplier-coefficient values employedtherein) are controlled or otherwise set by the microprocessordetermining the focal zone (or scanning range) from which thecorresponding analog scan data signal was obtained. Due to the timedelays involved during digital filtering (e.g. required to sample theentire length of the analog scan data signal associated with alaser-scanned bar code symbol and perform other digital signalprocessing operations required by the scan data signal processingmethod), the variable pass-band (VPB) control signals will need to bebuffered for a time period equal to the time delay. Thereafter, the VPBcontrol signal is linked up with the corresponding discrete scan datasignal being processed so that the digital pass-band filter can beoptimally tuned in accordance with the principles of the presentinvention. The discrete scan data signal is then differentiated using aderivative-type digital filter to produce a discrete first derivativesignal. The discrete first derivative signal is then subject to thederivative-type filter once again to produce the second derivativesignal. Both of these discrete derivative signals are then buffered inmemory a conventional manner. First derivative signal thresholds aregenerated for each discrete point in the discrete first derivativesignal.

In accordance with the method of the present invention, the discretefirst derivative signal is subject to a digital (FIR) filterimplementation of the bar code element detector, whereas the discretesecond derivative signal is subject to zero crossing detection. Inaddition, the following operations must be carried out in atime-synchronized manner: comparing the discrete first derivative signalwith its generated threshold values; enabling of the digital filterimplementation of the zero-crossing detector (using the output of thedigital filter implementation of the bar code element detector); gatingof the second derivative zero-crossings to the input of the digitaloutput level generator; and generating digital output values for thediscrete (binary-level) scan data signal. Thereafter, the discretebinary-level scan data signal can be “digitized” using digital signalprocessing techniques in order to produce a digital “time” count valuefor each of the first and second signal levels in the discrete binaryscan data signal. The digital count values can then be provided to aprogrammed decoder for decoding the scan data signal and producingsymbol character data string representative of the correspondinglaser-scanned bar code symbol. Alternatively, the generated discretebinary-level scan data signal can be converted back into acontinuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

Advantages Provided by the Analog Scan Data Signal Processor of thePresent Invention

One advantage provided by the scan data signal processor of the presentinvention is that the scan data signal processor has improvedsignal-to-noise ration (SNR) and dynamic range at its first derivativesignal thresholding stage, and improved SNR at its second-derivativezero-crossing detection stage, which results in a significant decreasein the effective beam diameter at each focal zone in the system. Inturn, this increases the length of each focal zone in the system, asshown in FIGS. 5A and 5B, which allows the system designer to (i)provide more overlap between adjacent focal zones or produce a laserscanning system with a larger overall depth of field; and (ii) produce alaser scanning system capable of scanning/resolving bar code symbolshaving narrower element widths and/or printed on substrates whose normalvector is disposed at large angles from the projection axis of scanningsystem.

In general,an excellent figure of merit for a laser scanning system isgiven by the ratio of (i) the laser beam cross-sectional dimension at afocal plane in the system; to (ii) the minimum element width of a barcode that can be accurately represented by the digital output of theanalog scan data signal processor when scanned at that focal plane. Asused hereinafter and in the claims to invention, this figure of meritprovided by the above-described ratio shall be referred to as the“Minimum Beam Dimension/Minimum Bar Width (MBD/MBW) Ratio”, oralternatively, the MBD/MBW figure of merit of a particular laserscanning system. The importance of this figure is revealed by the factthat a laser scanning system having a MBD/MBW ratio greater than unitycan resolve (or read) a bar code symbol with a laser beam having aminimum beam dimension greater than the minimum width of the bar codeelements in the scanned symbol. The greater the MBD/MBW ratio, thehigher the resolution of the bar code symbols that can be read by thesystem without increasing the output power of VLD producing the laserscanning beams in the system. Thus an increase in the MBD/MBW ratio of asystem implies that the less laser beam power is required to produce adigital scan data signal by virtue of an increase in SNR within theanalog scan data signal process of the system. By reducing the effectsof substrate noise in the laser scanning system of the presentinvention, it has been possible to use (or define) a smallercross-section of the laser scanning beams in the system, containing lesslaser power, to scan smaller bar code symbol elements without reducingthe overall SNR of the scan data signal processor.

As shown in the table of FIG. 5C, the MBD of the laser scanning beams ateach focal plane in the system of FIG. 1 are presented along the MBW ofbar code symbols that have been correctly read by the system at suchfocal planes. As indicated by the data in this table, the MBD/MDW ratiofor this system in each of its five focal zones is 2.0. Notably, thisfigure of merit was achieved using the analog signal processor shown inFIGS. 4—1 through 4H4, but without installation of the time-domainsubstrate noise filter 40 shown in FIG. 4A. With the substrate noisefilter installed in the signal processor, it is expected that theMBD/MBW ratio for each of the focal zones in the system of FIG. 1 willbe significantly increased to at least 2.1 or greater, thus providingfor the first time in history, a laser scanning bar code symbol readerhaving a MBD/MBW ratio comparable to that of conventional single focalzone polygonal-type laser scanning systems. It is understood that thelaser scanning system, and scan data signal processor of theillustrative embodiments may be modified in a variety of ways which willbecome readily apparent to those skilled in the art of having thebenefit of the novel teachings disclosed herein. All such modificationsand variations of the illustrative embodiments thereof shall be deemedto be within the scope and spirit of the present invention as defined bythe Claims to Invention appended hereto.

What is claimed is:
 1. A laser scanning system comprising: a laser scanning mechanism for producing a laser beam within a plurality of focal zones and scanning said laser beam over a bar code symbol, and producing an analog scan data signal indicative of the intensity of light reflected from said scanned bar code symbol; a control signal producing mechanism for producing a control signal indicative of the focal distance of the scanned laser beam at each instant in time of scanning system operation; a scan data signal processor for processing said analog scan data signal; wherein said scan data signal processor includes a plurality of first derivative signal pass-band filter structures that are electronically-switched into operation in response to said control signal at each instant in time of scanning system operation.
 2. A laser scanning system comprising: a laser scanning mechanism for producing a laser beam within a plurality of focal zones and scanning said laser beam over a bar code symbol, and automatically producing an analog scan data signal indicative of the intensity of light reflected from said scanned bar code symbol; and a scan data signal processor for processing said analog scan data signal, said scan data signal processor including a preamplification circuit for preamplifying said analog scan data signal produced in response to scanning a bar code symbol by said scanned laser beam within one of said plurality of focal zones, a first derivative signal generating circuit for deriving a first derivative signal from said preamplified analog scan data signal, a variable pass-band filter having a plurality of different pass-band filter structures having different pass-band filter characteristics for processing a signal selected from the group consisting of said preamplified analog scan data signal and said first derivative signal, a control signal generation circuit for generating a control signal indicative of the focal distance of the scanned laser beam producing said analog scan data signal provided to said preamplification circuit, and a filter control circuit for electronically switching into operation selected pass-band filter structures using said control signal, thereby automatically tuning said scan data signal processor to an optimum setting for the focal zone being scanned at each given moment of operation of said laser scanning system.
 3. The laser scanning system of claim 2, wherein an optical device is used to produce said laser beam and said control signal generation circuit comprises a mechanism for analyzing said optical device during laser scanning so as to determine the focal distance of said scanned laser beam.
 4. A laser scanning system comprising: a laser scanning mechanism for producing a laser beam within a plurality of predefined focal zones and scanning said laser beam over a bar code symbol, and automatically producing an analog scan data signal indicative of the intensity of light reflected from said scanned bar code symbol; and a scan data signal processor for processing said analog scan data signal, said scan data signal processor including a plurality of different pass-band filters, wherein one of said pass-band filters is dynamically switched into operation for pass-band filtering the analog scan data signal produced by said scanned laser beam scanning a bar code symbol within one of said predefined focal zones so as to filter out spectral components of paper noise residing outside the frequency spectrum of said analog scan data signal produced in response to scanning said bar code symbol within said predefined focal zone.
 5. The laser scanning system of claim 4, wherein said laser scanning mechanism is selected from the group consisting of a holographic laser scanning mechanism, a polygonal-type laser scanning mechanism, and any other type laser scanning mechanism capable of generating a laser scanning pattern having multiple focal zones.
 6. A multi-focal zone laser scanning system comprising: a laser beam scanning mechanism for (i) producing a laser scanning beam, (ii) focusing said produced laser beam within a scanning volume having a depth of field and a plurality of predefined focal zones, (iii) scanning said focused laser beam across a bar code symbol within said depth of field, (iv) collecting a laser light signal produced by said scanned laser beam reflecting off said bar code symbol, (v) detecting said collected laser light signal, and (vi) producing an analog scan data signal corresponding to the detected laser light intensity and having a frequency bandwidth determined by the speed of the laser beam across the scanned bar code symbol and the structure of said scanned bar code symbol; a control signal generator for dynamically generating a control signal indicative of the predefined focal zone in which said scanned bar code symbol resides at any instant in time when scanned by said laser beam during said laser scanning operations; and an analog scan data signal processor including a first derivative signal generator for receiving said analog scan data signal and generating a first derivative signal representative of the first time-derivative of said analog scan data signal; a plurality of pass-band filter structures, each one of said plurality of pass-band filter structures being preassigned to one of said predefined focal zones and having frequency response characteristics for optimally filtering the first derivative signal produced when scanning said bar code symbol by said scanned laser beam within said predefined focal zone; and control circuitry, responsive to the control signal generated by said control signal generator, for dynamically switching one of said plurality of pass-band filter structures into operation so that the first derivative signal produced in response to said laser beam scanning a bar code symbol within one of said predefined focal zones is optimally filtered by said dynamically-switched pass-band filter structure preassigned to said predefined focal zone.
 7. The multi-focal laser scanning system of claim 6, wherein at instant in time said dynamically switched pass-band filter structure filters out the spectral components of paper noise residing outside the frequency bandwidth of said first derivative signal, produced by said laser beam when scanning said bar code symbol within said predefined focal zone.
 8. The multi-focal laser scanning system of claim 6, wherein said laser beam scanning mechanism comprises a holographic scanning disc for supporting a plurality of holographic scanning facets for scanning said produced laser beam, wherein each said holographic scanning facet has focal length which falls within one of said predefined focal zones, and wherein said control signal generator comprises a holographic scanning facet detector for detecting the holographic scanning facet which generates said laser scanning beam at any instant in time, and automatically produces said control signal.
 9. The multi-focal laser scanning system of claim 6, wherein said laser beam scanning mechanism comprises a polygonal-type laser beam scanning element for scanning said laser beam during laser scanning operations, and a variable laser beam focusing mechanism for focusing said produced laser beam to one of said predefined focal zones in response to said control signal; and wherein said control signal generator comprises an object measurement device for automatically measuring a physical dimension of the object on which said bar code resides, and automatically generating said control signal.
 10. The multi-focal laser scanning system of claim 6, wherein said physical dimension is the height of said object measured relative to a surface on which said object is supported during laser scanning operations.
 11. A multi-focal zone laser scanning system comprising: a laser beam scanning mechanism for (i) producing a laser scanning beam, (ii) focusing said produced laser beam within a scanning volume having a depth of field and a plurality of predefined focal zones, (iii) scanning said focused laser beam across a bar code symbol within said depth of field, (iv) collecting a laser light signal produced by said scanned laser beam reflecting off said bar code symbol, (v) detecting said collected laser light signal, and (vi) producing an analog scan data signal corresponding to the detected laser light intensity and having a frequency bandwidth determined by the speed of the laser beam across the scanned bar code symbol and the structure of said scanned bar code symbol; a control signal generator for dynamically generating a control signal indicative of the predefined focal zone in which said scanned bar code symbol resides at any instant in time when scanned by said laser beam during said laser scanning operations; a scan data signal processor for processing said analog scan data signal, said analog scan data signal processor including a plurality of pass-band filter structures, each one of said plurality of pass-band filter structures being preassigned to one of said predefined focal zones and having frequency response characteristics for optimally filtering the analog scan data signal produced when scanning said bar code symbol by said scanned laser beam within said predefined focal zone; and control circuitry, responsive to the control signal generated by said control signal generator, for dynamically switching one of said plurality of pass-band filter structures into operation so that the analog scan data signal produced in response to said laser beam scanning a bar code symbol within one of said predefined focal zones is optimally filtered by said dynamically-switched pass-band filter structure preassigned to said predefined focal zone.
 12. The multi-focal laser scanning system of claim 11, wherein at instant in time said dynamically switched pass-band filter structure filters out the spectral components of paper noise residing outside the frequency bandwidth of said first derivative signal, produced by said laser beam when scanning said bar code symbol within said predefined focal zone.
 13. The multi-focal laser scanning system of claim 11, wherein said laser beam scanning mechanism comprises a holographic scanning disc for supporting a plurality of holographic scanning facets for scanning said produced laser beam, wherein each said holographic scanning facet has focal length which falls within one of said predefined focal zones, and wherein said control signal generator comprises a holographic scanning facet detector for detecting the holographic scanning facet which generates said laser scanning beam at any instant in time, and automatically produces said control signal.
 14. The multi-focal laser scanning system of claim 11, wherein said laser beam scanning mechanism comprises a polygonal-type laser beam scanning element for scanning said laser beam during laser scanning operations, and a variable laser beam focusing mechanism for focusing said produced laser beam to one of said predefined focal zones in response to said control signal; and wherein said control signal generator comprises an object measurement device for automatically measuring a physical dimension of the object on which said bar code symbol resides, and automatically generating said control signal.
 15. The multi-focal laser scanning system of claim 14, wherein said physical dimension is the height of said object measured relative to a surface on which said object is supported during laser scanning operations. 